Redistricting should be a way of ensuring your vote counts. If all districts have roughly the same number of people in them and are drawn to respect natural communities -- neighborhoods where people share a heritage, work in the same industry, or just generally feel tied to their neighbors -- voters have a chance to be represented by politicians who represent their areas' collective interests.
To that end, states are required to redraw lines for districts, all the way from Congress to county boards of supervisors, every 10 years to reflect demographic changes.
But that's where theory meets the harsh reality. Instead of voters choosing politicians, redistricting at its worst lets politicians choose voters.
Communities can have their influence diluted or overly concentrated by line-drawers interested in partisan gain, limiting minorities' influence, or pleasing powerful interests. (See our earlier story, The Hidden Hands in Redistricting.) The right lines can all but guarantee an incumbent a decade's worth of electoral success, or alternatively can help send others into retirement.
Such shenanigans persist, despite the 1965 Voting Rights Act and subsequent legal decisions meant to limit them. Indeed, increased mapping technology and know-how have allowed for ever more subtle manipulation of district lines.