Hi,
One more point - you have to remember that a BAB is purely the number that you get by adding together the + "to hit" numbers from your classes - it doesn't include (for example) plusses from Dexterity (or Strength in D&D) etc. This is important because:
1/ You get an extra attack per round whenever your BAB exceeds a multiple of 5 i.e. at +6 you get two attacks, one at BAB +6 and one at BAB +1, at +11 you get three, at +11, +6 and +1 etc.
You add your other bonuses onto these numbers.
2/ Prestige classes will often have a prerequisite of BAB +n. e.g. Big Game Hunter has a requirement of BAB +5, so you won't be able to get into the prestige class until you're at least 5th level, even if your DEX is 20...
cheers,
Mark