What is Twenty20 or T20 in cricket?
Twenty20 or T20 for short is a shortened version of the game of cricket in which each side faces a maximum of 20 overs. The game typically lasts about 4 hours with a quick turn between innings unlike other forms of the game, thus increasing its viewership and popularity. 

This format was introduced by the England Cricket Board in an inter-county competition back in 2003. Since then, its become really popular with several international competitions and country wide leagues across all the major cricket playing nations world wide