What does AT&T exclusive mean?

I want to buy a case from speck but the one I want says it is an AT&T exclusive. Will it still fit my iPhone 4 that I bought from Verizon?

The AT&T iPhone has a bit of different design than the iPhone from Verizon (the antenna is in a different place), but the case should still fit just fine.

Don't worry about this
they are the same size.
it must fit your iphone 4