Gerrymandering is the process of drawing boundaries for legislative districts so as to maximize the number of safe seats for a particular (or, in some cases, both parties), while minimizing the number of seats which may be subject to a competitive race. By eliminating competition, the world is made safe for incumbents. Further, it tends to reduce the number of moderates in the legislature, since there is no need to attempt to attract crossover voters from the other party in the general election.

The word derives from a long-ago Thomas Nast editorial cartoon where a particular district resembled a salamander and was blamed on then-Governor Gerry -- hence, a "Gerrymander".

Here's a site that appears to be Republican-slanted, but which also appears to cover the subject pretty well, based on a brief skim of the contents.