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:
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internationalization: they used new terminology, rather than using language tied to US legal concepts
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patents: they specifically address patents (including the Microsoft/Novell issue noted in another answer)
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“Tivo-ization”: they address the restrictions (like Tivo’s) in consumer products that take away, though hardware, the ability to modify the software
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DRM: they address digital rights management (which they call digital restrictions management)
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compatibility: they addressed compatibility with some other open source licenses
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termination: they addressed specifically what happens if the license is violated and the cure of violations
I agree with the comment about consulting a lawyer (one who knows about software license issues, though). In doing these things (and more), they more than doubled the length of the GPL. GPL 3 is many things, and one of them is that it is a very complex, technical legal document.