What is gerrymandering? The simplest explanation would be: instead of voters choosing their politicians, politicians choose their voters.
Confused? Take a look at the following video from Ezra Klein.
Basically, politicians redraw the electoral district boundaries so as to maximize their support in the newly created district. They split the state into as many districts as possible, each tailored based on the voter support they receive at the particular territory. The obvious outcome is that you can get more seats for less votes (a typical quirk of the first-past-the-post electoral system - just ask the LibDems in the UK). To get a better sense of what it is take a look at the following few maps of the US Congressional Districts:
|California, District 38|
|Florida, District 5|
|Illinois, District 4|
|Top three: North Carolina, D-12, Florida, D-5 (again), Pennsylvania, D-7|
Bottom three: Maryland D-3, North Carolina D-1, Texas D-33
There's many more, less obvious, examples but one thing is clear - in these districts politicians literally choose their voters. And, as said, the outcome is getting more seats with less votes. For example in the 2012 US House elections, Democratic candidates won 1.4 million more votes, but Republicans got more seats (234 to 201). Don't confuse this with the electoral college system for US Presidential elections, which gave Bush the Presidential victory over Gore even though Gore won the popular vote. (Btw, on that subject, take a look at a very interesting paper on the so-called butterfly ballot which might have swayed Florida, and hence the Presidency, towards Bush. Another example of "creativity" in designing electoral rules.)
Another consequence of gerrymandering is lack of accountability of Congressmen who are pretty much safe in their seats in a rigged electoral system. Without proper accountability, and after having to rely on a small group of essential supporters for re-election, democrats quickly turn into autocrats. Not by oppression or committing mass murders, but by corruption and theft. This is the central argument of the so-called selectorate theory, devised by four political scientists, Bueno De Mesquita, Smith, Morrow and Siverson. When the incentives to be accountable with the taxpayers' money are lacking, then politicians (regardless of which system they come from) have a tendency to act in a more corrupt manner. I recommend the book "The Logic of Political Survival" by the four authors, or even a popular version of the argument - "The Dictator's Handbook". I briefly covered part of their theory back in 2012.
So how does one prevent gerrymandering? The most obvious solution is to take the gerrymandering decisions out of the politicians' hands and assign it to courts or some nonpartisan commission. California has done some progress on that area with their California Citizens Redistricting Commission. Canada did it back in the 1960s, appointing an independent commission to redraw district boundaries. Down at the Vox webpage (from where the video was linked), they have presented a map of the US without gerrymandering, made by the Center for Range Voting. Nonpartisan initiatives such as these are exactly the sort of thing needed to prevent the electoral frauds and altogether the unfairness of the first-past-the-post system. And most importantly they increase the accountability of politicians in democracies, preventing them from turning into a certain Bell, CA city manager.