From Diablo Wiki
Raiding is a term used in many MMORPGs. It refers to large scale battles, generally ones against very powerful monsters or dungeons, that require multiple players working in coordination to defeat. Raiding is often the end game activity in MMOs such as Everquest and World of Warcraft, where the toughtest monsters who yielded the best items could only be obtained from very powerful enemies that required numerous (up to 40) players to team up to defeat them.
Raiding does not technically exist in Diablo III; all of the game is designed to be defeat-able by solo players.
- See the wikipedia entry for more details.