For who don't know what the ESRB is, the ESRB is a group in the US and Canada who rate games on Sexual, blood and gore, language and more. The ratings most often given are E, E10+, T, M and AO. Games granted the AO ratings are not allowed to be sold in US stores. But, how was the ESRB started?
Well, it was started by the game publishers of North America (Canada, and the US). When the first Mortal Kombat came out, it was so violent that is caused a controversy in the US whether or not the government should let this game be sold or not. This is in 1994 when 16 bit games where just becoming more popular over 8 bit. In 16 bit games, blood was more detailed and looked liked a fluid. This caused the US government to force the game publishers to create a system of ratings with in a year, if they failed to do so, it would be taken into the governments hands.
They then created the ESRB, Entertainment Software Rating Board. Depending on what was in the game. The ESRB would give the game a E, E10+, T, M, or if it is that violent, a AO rating. To learn more, go to the source.