Gerrymandering is a practice intended to establish a political advantage for a particular party or group by manipulating district boundaries.
Supreme Court cases require state legislatures to redraw districts every ten years (coinciding with the national Census) to ensure that legislative districts are roughly equal in population. In redrawing districts, officeholders can dilute the opposition party’s votes by “cracking” its voters across several districts, making it more difficult to construct a majority in any district, and can waste the opposition party’s vote by “packing” its voters into unnecessarily safe districts, reducing its capacity to compete in the remaining districts.

One of many extreme examples - Marylands 3rd district.

Gerrymandering is commonly blamed for polarization, but there are several studies which discredit that theory.
At "Does Gerrymandering Cause Polarization?" they "find very little for such a link."

Google search for Gerrymandering
This is the best explanation of gerrymandering you will ever see - The Washington Post

last updated 4 Feb 2014