There's different definitions, depending on the lifestyler you ask, but let's go with some generalized information:
BDSM: Bondage, Domination, Submission, Sadism and Masochism. There's other things in there too, but those are the basic things. Some of these things fall under sexual fetishes, others under personalities and relationship 'positions'. Sometimes they overlap. Within every single one of these words, there's thousands of things, definitions, terms and so on that could fill a book with information and interpretations, so it's impossible to encompass the complexity and variety within those four letters.
There's as many different levels of dedication to these things as there are people. Some find themselves just looking for a little kink in the bedroom, and finds a little joy in BDSM play, picking out a few kinks and fetishes for use within their sex life. On the other side of the scale, it's gone far beyond just the sex, and has turned into a lifestyle. Dominance and submission in a relationship, 24/7. Or Sadism and Masochism 24/7, not just within the bedroom.
Now, the above is very simplified, and doesn't even remotely encompass the 'true' scale of these things, but a lifestyler is simply one who lives the fetish, or lives the lifestyle. It's a larger part of their life, or the relationship they're in have large elements of either the D/s or S&M elements. Some even go so far as to become so submissive that they spend their entire day as a 'servant' to their Dom/Domme, who in turn spend their day making sure that the sub under their charge is well cared for and taken good care of in all things.
There's even some who go so far as to create legal slave contracts between them. That is taking things too far, in my own opinion, as it's touching on severe immorality and lack of human worth. However, it's still a free choice to begin with, as long as there's also a psyche evaluation prior to the contract signing.
Anyway, a lifestyler is thus someone who lives the BDSM life. Or a related but necessarily BDSM 'kink' becomes a large part of their life, and thus lifestyle.
Now, this'll have to do for now, on the question of lifestylers. But any questions on more specific matters would be more than welcome. It's just a little hard to write answers to such generalized questions which basically says "Tell me EVERYTHING about this.", as the answers will inevitably lack half the information needed to truly understand it.