Eben Moglen (Free Software Foundation) sums up the differences:
> But most proprietary software companies want more power than copyright alone gives them. These companies say their software is “licensed” to consumers, but the license contains obligations that copyright law knows nothing about. Software you're not allowed to understand, for example, often requires you to agree not to decompile it. Copyright law doesn't prohibit decompilation, the prohibition is just a contract term you agree to as a condition of getting the software when you buy the product under shrink wrap in a store, or accept a “clickwrap license” on line. [..]
> The GPL, on the other hand, subtracts from copyright rather than adding to it. [..] Copyright grants publishers power to forbid users to exercise rights to copy, modify, and distribute that we believe all users should have; the GPL thus relaxes almost all the restrictions of the copyright system. The only thing we absolutely require is that anyone distributing GPL'd works or works made from GPL'd works distribute in turn under GPL. [..]