To insure a Congressional seat for a specific party or candidate, districts are sometimes divided up in such a way as to strongly favor just one party. Usually, it is Democratic districts that are gerrymandered in a most unconventional and corrupt manner.
Originally, states and communities were supposed to be divided into "wards" of equal numbers of people - in blocks. But as corruption overtook Congress, some politicians began moving the boundaries of their districts to incorporate only those areas that were primarily among his party. In other words, the new boundaries would incorporate primarily Democrats while keeping out Republicans, thereby insuring the seat for Democrats.
To help you see exactly what they do, here are the 9 of the top gerrymandered districts in the country. As you can see, they go to great lengths in some cases to "cut out" republican areas. In only one case is the gerrymandered district a Republican district.
As Americans, we should be demanding that our Congress stop messing with districts, and remove all existing gerrymandering, which only breeds corruption. And there is already far too much of that in Wshington. All districts should, as best as it can be done, put back into "block" form. Only in that way can we insure fair elections that are not unfairly tilted in favor of any one party.