BDSM is a convenient abbreviation for most of the interesting activities discussed in alt.sex.bondage. It's so convenient that it packs six initials into four letters B&D/D&S/S&M => BDSM. It's generally understood to include related
activities/phenomena that don't fit strictly into any of those three categories. 
An "umbrella term" like this is useful because so few actual (as opposed to
theoretical) activities fit into _only_ one category. 

Bondage deals with tying people up (or being tied up). Or chaining them up, or
restraining them with straps, or straight jackets, or ... well, you get the idea, no?
In theory it can be enjoyed simply for its own sake -- the sensations and images
of it. In fact, some people do enjoy bondage as bondage, without any interest in
D&S or S&M, but far more people find it pushes their D&S buttons at the same
time, or use it only for the D&S aspects, or combine it with D&S and/or S&M. 

Dominance and submission deals with exchange of power, trust, obedience,
role-playing, "slavery" ... one person submitting to the commands of another.
Like bondage, it can exist as a separate phenomenon, but it's likely to
incorporate the others. Bondage may be used to enhance the feeling of
submission. Pain-play (i.e. S&M) may be used to emphasize the position the
submissive is in or as punishments for disobedience. S&M sort of stands for
"sadism and masochism", but not quite the same way the psychiatric
establishment uses those terms. So it's less confusing to keep the phrase tidily
together as "S&M". 

S&M involves strong sensations. It's associated with pain, in particular, in most
people's minds, but in fact pain is only one class of sensations used.
Furthermore, some stimuli which would ordinarily be perceived as pain are not
perceived as pain by some participants when in an S&M headspace! (Note that I
said "some".) While I don't have statistics on this, it's my impression that S&M is
the one phenomenon of these most likely to occur without the others.
Nonetheless, it is quite common for one's interest in S&M to be in the context of
bondage or D&S (them pain makes it so very much clearer that one can't get
away because one is tied up, for example) or simply _alongside_ an interest in
bondage and/or D&S. 

Interestingly, while most "vanilla" (i.e. not-into-BDSM) people do not consider
tickling to be a BDSM activity, many BDSM folks do. 



More terminology

Some people like to tie people up, whip people, or give orders. Others like to be
tied up, like to be spanked or whipped, or like to obey. Because so many of the
words one might use to describe these preferences seem specific to just one
aspect of BDSM, push people's buttons, or only fit the ways some people play,
folks in the scene use the generic terms "top" and "bottom". (Note that these
words have a different meaning in gay male culture, if I'm not mistaken.) 
In bondage, a top likes to tie up bottoms. In S/M, a top likes to provide strong
stimulation (pain or otherwise) to a bottom. In D&S, a top orders or controls a
bottom. A "switch" is someone who enjoys being both a top and a bottom. 

Note that it's not always the top who's in control of things -- in fact, much less
often than the other way around! For example, a bottom might ask to be tied up,
and his or her top might decide to honour that request, asking the bottom if there
were any particular things he or she wanted the top to do to him or her tonight.
Also, many people use "safewords", code phrases that mean, "I'm not just
playing, I really need you to stop." If a couple uses a safeword, the bottom can
stop the current activity by using the safeword. 

Some people claim that the bottom is always the one who's really in control, no
matter how things look. They're mostly right, but things can get more
complicated.