I'll do what I can to answer your questions, dude.
Bleach is an anime series with like a thousand episodes, lol. Check out a wiki to get an idea about it. (That might be obvious, though, heh)
BESM is a table-top roleplaying game system (I believe). It stand for Big Eyes Small Mouth, and is apparently used for RP'ing anime or anime-like scenarios. I had never heard of it before I joined this RP, so if that's wrong, sue me, lol. I have been told that my lack of knowledge of BESM rules is not a problem here (I'll get clued in as necessary, apparently), so don't let that deter you from being interested, if saikotsu is still accepting players.
Single Shots: if you are referring to the so-called "One-Shots" RP section,then I think I know what you mean. At the top of the One-Shot section is a single thread in which everybody posts their requests for a one-shot. You simply have to scroll through the many pages to find something you'd be interested in. Follow this link to get there.
The usual point of a One-Shotter, from what I've been told, is to either find someone to RP a rather specific situation or scene with you, which will most likely end after that scene is finished (unless the RP'ers decide to keep it going). Also, apparently you can RP a one-shot with somebody to see if your styles and expectations mesh well before you commit to a more long-term RP.
Hope I answered your questions accurately.