You get a attack with your full BAB with your primary weapon(s), and an attack at -5 BAB with only 1/2 str to damage with your secondary weapon(s).
If your primary attack was "Claw", you'd get Claw Claw (assuming the animal has 2 claw attacks in their Full Attack Sequence, I think most of em do), at Full BAB and full str damage, and Bite at -5 to that, with 1/2 str damage.
If you only have one natural weapon, you do 1 1/2 times str damage with it.
If you're an animal that can use weapons (Ape, for example), and you were holding a weapon (Sword), you'd use the sword naturally (one swing for BAB 1~5, two swings for 6~10, etc), and get an attack out of each of your natural weapons at -5, and at 1/2 str to damage.
If you have the 3.5 books, look at page 299.
Note that Druid's Wild Shape description implies that other classes are not proficient in the natural weapons that they polymorph into.