SOPA and PIPA are just one part of a decades old trend of copy protection legislation
Of all the SOPA and PIPA explanations I've heard, probably one of the most concise and well delivered was given by Clay Shirky as posted on TED (It's worth a watch and just under 14 minutes). In addition to being a lot more informative then some of the knee jerk commentary on the issue, he gets around to the bigger issue at hand. That these two bills are merely just one more step in a process that has been going on for decades, and that will continue to go on for years still as companies keep trying to pass this sort of legislation. Even with those two bills currently shelved, we now have ACTA being designed behind closed doors over in Europe. There are also fears that in the future, parts of these policies will be pushed through attached to other bills or enacted through foreign policy, where public awareness and legislative accountability will be side stepped completely.