Krewes are
clubs which run
Mardi Gras festivities, usually a
parade or a
ball. In
New Orleans, there are dozens of krewes which each put on some sort of event in the weeks close to Mardi Gras.
The Krewe of Rex puts on the main parade of Mardi Gras day.
Krewes have names based on ancient times and mythology (Krewes of Babylon, Proteus, Zeus, etc), geography (Mid-City, Carrollton, etc), and other more random things (such as Krewe of Elvis).