PlayStation's Ultimate Gaming Glossary of popular gaming terminology.
Wikipedia's Glossary of Video Game Terms, including slang.
The Entertainment Software Rating Board, or ESRB, is a self-regulatory body established in 1994 by the Entertainment Software Association (formerly called the Interactive Digital Software Association) to rate video games based on appropriateness for different maturity levels. Essentially, it was formed by the video game industry itself in an effort to regulate video games and reduce youth exposure to violent, sexual, graphic, or other mature content, under the threat of government intervention. It has seen great success and has become the industry standard in North America (United States, Canada, and Mexico) for rating video game titles by age-appropriateness.
You can find more information on the ESRB from the video below, and on their official website: https://www.esrb.org/