The history. The purpose. The sound effects. The enormous write-up over on Halo Waypoint (linked below) has everything a series fan - or firearm fetishist - could ever want with regards to humanity's weapons of choice.
And if you don't feel like reading? Here's every UNSC gun in the game, shooting things in the face.
The Halo Bulletin: 8.8.12 [Halo]