I am a Domme, that is I have a relationship with a (lovely) submissive woman where, within negotiated limits, she is under my control. Often denoted as a D/s relationship (D for dominant, s for submissive). This relationship suits both of us - meeting emotional and sexual needs of each. I get: a huge emotional charge of erotic feelings and creative energy, as well as receiving heaps of love, service and devotion - she gets: a lot of care and attention; excitement; a "letting-go" that comes of not having to decide anything; and *intense* sexual/endorphin highs. These emotions are extensions of common emotions experienced with sex and love - when in love one can often feel waves of submissiveness, wanting to do ridiculous things to prove/show ones devotion - similarly one can be hit with impulses to posses and control the person you desire. D/s is simply when these sorts of emotion are long-standing and openly expressed.
Many D/s relationships (but not all) involve fetish elements, that is involving some aspects like: pain; humiliation, control, bondage, exhibitionism, sexually explicit clothing, rules as well as fantasy recreations of such as: imprisonment, abduction, and forced sex. These elements are generally what is indicated by the initials BDSM - but this is another broad, catch-all phrase. This is due to the fact both D/s and BDSM are associated with a related series of human responses and emotions, and that to do BDSM role play involves control. However I think it is also due to the fact that a D/s structure to a relationship facilitates the exploration of taboo areas - it provides a framework and an emotional license. The D partner is freed from normal constraints of what is socially acceptable as a result of being in command, and the s partner is free to experience taboo sensations without feeling guilty about it. It is almost universal for there to be implicit or explicit negotiation about what sort of things the s wants to be "forced" to do.
So although D/s and BDSM are closely associated they are not necessarily connected. There are D/s relationships that do not involve any fetish elements at all, indeed some that do not involve sexual aspects at all. Likewise there are people who use BDSM in their sex or role play without there being any spill over into their day-to-day relationship - they are as assertive in the workplace or with their children as anyone, but submissive in the bedroom (or the reverse). In fact the later (occasionally BDSM play) is far more common in RL than a longer-term D/s structured relationship. In SL the situation is a little bit different since people come to SL to play (in this sense). Thus within SL there are many who enter a longer-term D/s relationship in order to experience BDSM, because for them SL is all play. There are others for whom SL is a part of their "serious" life in some sense - they don’t play but live SL - for them they may choose to have occasional role-play sessions within SL, either with casual partners or within distinct "scenes" with longer-term partners. There are a few, like me, who both live SL and enter a longer-term D/s relationship - this involves a considerable emotional and time commitment, not to mention a responsibility to the safety of their partner(s). There are even a few people who are in long-term ("lifestyle") D/s relationships in RL, but come into SL to explore further!
Yes we are playing with some dark emotions here! Our interaction includes things that are totally taboo in normal life - that would be simply wrong if it were not for the fact that we *both* enjoy it and it is fundamentally consensual (that is either of us could just stop at any time and we are not forced into this by economic or other circumstances). It is also not without psychological dangers - this is very like an "extreme sport", you get the adrenaline rush and excitement but you also need good preparation and safeguards. It is for this reason that some generally agreed norms and procedures have been developed to allow for the exploration of these kinds of experiences at the same time as keeping everybody from serious harm. The three main principles have been given the acronym: SSC: Safe, Sane and Consensual - these are overriding principles, that should be taken above any other considerations (such as fun or desire). Any practice should be safe - that is not cause any permanent and/or serious injury to the body of any participant; it should protect sanity - that is not be injurious to the mental health of any participant and it should be fundamentally consensual.
The last of the above three is a little tricky to implement since in play "no don't spank me" might actually really mean "please spank me"! There are several main ways to get around this problem and ensure consent is really and currently given. The first of these is the use of a formal agreement about the limits of control and the kinds of things that the s wants/is willing/is unwilling to experience. There is a common and intimidating list of BDSM practices which some Dommes get their subs to fill in before they start playing/interacting seriously, but it is more common for people to just negotiate in rough terms these likes/dislikes/limitations. The second is the use of a safeword - this is a special word (not "no" or "stop" for obvious reasons) that can be used by either s or D to cause a cease to all activities and initiate communication between partners. This gives a safe guard to both the s (they can stop play if it goes too far) but also to the D partner who can have some reassurance that the sub begging for release before them is actually enjoying it and not really wanting release. Special extensions of this are needed when gags etc are used, but in SL the use of private IM between D and s is an ideal channel for this sort of message and negotiation. The third mechanism is that of an off-line communication medium whereby partner can honestly communicate how they felt about what went on, via a journal accessible to the other, or in the case of SL emails.
"OK, OK so what do D/s people actually do, sexually speaking?" is the obvious question. Well I can't tell you in general, any more than I could tell you what people do together in any other intimate way. I have talked to people, read what people have written, but really I can only talk authoritatively about my own experience. What I do may well not be representative of what others do! I am not aware of any surveys similar to those of normal (sometimes called "vanilla" by D/s folk) sexual behavior. Just like any loving/sexual behavior it is very personal and intimate - depending more than anything on the person you are with, not the actions you take. So I am afraid if you are reading this for intimate details, you are going to be disappointed - retelling what I have done without the loving context it is done in would be simply misleading. Compare the case to trying to understand sex for the first time and only having a dry telling of its mechanics.
For most of my time in SL my sexual relationships have been entirely non-BDSM, that is straight forward lesbian love affairs. I am extremely sensitive to my partner's wishes - there is nothing that gives me deeper pleasure than loving the whole person I care about, including their sexuality. There is a deep joy to be had in satisfying ones partner's sexual fantasies/needs far more than they had ever expected. Loving people like this can make them feel whole - at last they can be loved for who they are as a complete person, including any sexuality that they had previously "boxed-up" in their head as socially unacceptable. Thus I have done some things that might be considered more extreme for the people I care about in SL, including being their sub (which I loved), letting them suck my blood, make love in a closed coffin, and pulling them through public places naked and chained. There are only three rules when it comes to sex: don't harm people, respect and care for your partner, and (if you able) try to satisfy your partner's needs. If an action gives pleasure (especially mutual pleasure), then it can be used to convey love and care. BDSM can be very much a way of both caring and "making love".
Like other sex this can be used for good or ill. There are some selfish, nasty or simply sick people out there being Dommes (or subs) and causing emotional distress to others or themselves - but this is the same as with other relationships. One needs to choose one's partners with care - one does not have sex with anyone (if you are wise) - this is doubly true with D/s or BDSM. Taking time to get to know someone before committing is always a good idea - and if the person is not willing to do this, to put in time and energy developing a deeper relationship then this tells you a lot about that person - D/s is not different in these fundermentals than any other relationship, but based on fundermental respect, care and communication.
I do not pretend that I am acting altruistically in this - I do enjoy domination - otherwise I could not do it well. You can no more pretend to be a Domme or a sub that you can pretend to be a lesbian - it is something that is built into you or not (at least by the time you are an adult). However, because my sexuality is intimately bound up with loving and caring, I can only enjoy it if my partner is as well (just like any sex). I presently have both a normal lover in SL and a devoted sub. I am committed to both these relationships, to caring and loving them in their different ways, although if my lover asked me to, I would drop the D/s relationship because my love for her is paramount.
I get great pleasure and joy from caring for and dominating my sub and she gets great pleasure for being cared for and submitting to me. It suits us both. It is a vehicle for our care and love (respectively). It is good. If you want to know whether you have any similar feelings in you, try this simple thing with your partner - try dancing somewhere romantic (an intimate salsa, tango or slow dance is best) but where one of you is handcuffed (the dominatech cuffs available at slexchange.com or Venus (224, 242, 28) are simplest and most effect here because they work well with dance animations) - see how you feel. I have yet to find anyone who does not find it deeply erotic - it is definitely one of my favorites!
If you are genuinely interested in this, I suggest you DO NOT type BDSM into Google and keep your eyes open! That will get you lots of images designed to titillate male fantasy but will not help you understand real D/s and BDSM sexuality. Imagine trying to understand sex by just looking at pornography - it is the same with BDSM.
There are two good books about BDSM, called "The New Topping Book" and "The New Bottoming Book" by Dossie Easton and Janet Hardy that go into this in detail. They are mostly concerned with RL and have only one chapter on using the internet and longer-term D/s relationships but still are marked out as being insightful, sensible and humane. (available at amazon, e.g. http://www.amazon.com/New-Topping-Book-Dossie-Easton/dp/ and http://www.amazon.com/New-Bottoming-Book-Janet-Hardy/dp/ similar bookshops).
I am not going to show pictures of people involved in D/s or BDSM here, for fear of attracting merely prurient interest. However you can find picks from SL on flickr using this search (or similar): http://www.flickr.com/search/?q=secondlife+bdsm
If you think the above is (as Freud characterized it) simply pathological, I suggest you read an article called "Psychology And BDSM: Pathology Or Individual Difference?" at http://ipgcounseling.com/psychology_and_bdsm.html
If you feel this is a part of you (either D or s or both), then I suggest you gradually experiment with someone you love and trust - talking about your experiences together. There are various places in SL that claim to teach D/s and BDSM, but I cannot say that I recommend them since they can only really deal with the mechanics and not the emotional component (which is, after all, the point) - they tend to reinforce stereotypes rather than promote good D/s relationships.