for those without the books
You gain a bonus to your attacks every odd level I believe, not to your base attack bonus, just you're attacks. So at level 20 your number of attacks per turn are locked. You gain +1 to your saves at every even level, so the difference between your high save and low save won't grow any more. There is a whole new feat list that becomes available. You still gain hit dice like usual and caster levels continue to go up, but if you continue to a level 21 wizard you gain 1 tenth level spell, another level I believe adds a level 11th spell and so forth, that I'd have to check into.
Some class abilities continue to grow, like paladin cure disease, Ranger favored enemy and rogue sneak attacks. Some abilities get replaced by bonus feats every couple of levels, like the enhanced unarmed damage of monks and the lore ability of the loremaster.
Any questions not answered?