Base Attack Bonus

Here are the base attack bonus, per class:

Level	Barb, Knight, Ranger	Rogue, Priest, Druid	Mage
1	1	0	0
2	2	1	1
3	3	2	1
4	4	3	2
5	5	3	2
6	6/1	4	3
7	7/2	5	3
8	8/3	6/1	4
9	9/4	6/1	4
10	10/5	7/2	5
11	11/6/1	8/3	5
12	12/7/2	9/4	6/1
13	13/8/3	9/4	6/1
14	14/9/4	10/5	7/2
15	15/10/5	11/6/1	7/2
16	16/11/6/1	12/7/2	8/3
17	17/12/7/2	12/7/2	8/3
18	18/13/8/3	13/8/3	9/4
19	19/14/9/4	14/9/4	9/4
20	20/15/10/5	15/10/5	10/5

The base attack bonus is pretty simple to understand. If you have an attack bonus of 15/10/5, you have three attacks per round. The first will be at +15, the second at +10 and the third at +5 (the STR or DEX bonus is applied as well). The final attack bonus is equal to your base attack bonus, plus your STR or DEX bonus (depends on the weapon).

Note: For melee weapons, your final attack bonus = Base attack bonus + STR bonus
For ranged weapons, your final attack bonus = Base attack bonus + DEX bonus