The page linked to in another answer is a good source, but a lot to read. Here is a short list of some the major differences: internationalization...

This link also highlight the differences between GPLv2 and GPLv3. Content:

This page highlights the differences, but it's still quite a lot to read through.

In (not entirely) cynical terms, the reason for the v3 license was Microsoft's patent deal with Novell. In reality, you should always consult a la...