[flyingfences]

The authors have written up a summary [0] of the differences: in short, 3e reflects the changes in typical computer hardware over the last decade or two.

Note also that there are two versions of the third edition: the standard edition (colorfully striped shape on the cover) and the international edition (globe made of circuit boards on the cover) which is quite a bit cheaper. The contents of the chapters are the same, but the exercises at the ends of the chapters have been rewritten, by the publisher's lackeys, not by the original authors, for the international edition, very poorly by all accounts.

[0] http://csapp.cs.cmu.edu/3e/changes3e.html