Base attack bonus (BAB) defines the number of normal attacks per round a character has, as well as an amount added to the attack bonus of each of these attacks. It is expressed as a sequence of bonuses, as in "+12/+7/+2", which would mean three attacks per round, with the first attack roll getting +12 from BAB, the second +7, and the third +2.
BAB is based only on base attack, which in turn is based solely on class; modifiers from strength, feats, etc. do not contribute to the base attack progression.
The first attack in a round always has a modifier equal to the base attack.
Up to character level 20, an additional attack per round is obtained when the lowest bonus in the BAB sequence would otherwise be +6. This additional attack starts with a +1 modifier, which then remains 5 less than the prior modifier.
Thus for any base attack, the BAB progression is the base attack, followed by a sequence of modifiers decreasing by 5.
An exception, however, is made for monks that are fighting unarmed or with a kama; these characters use the special unarmed base attack bonus progression instead. (In brief, monks use -3 instead of -5.)
In either case, though, additional attacks are not gained after level 20. The bonuses continue to increase, but the number of bonuses is fixed at what it was at level 20.
Base attack and BAB
add to row for level 20 BA
While individual class levels do not matter beyond their contribution to base attack, the following table may be a useful reference for some.