Short for Console Variable, a cvar is a variable that can be set via typing it into the console, followed by a valid value. (For example "g_gametype 0" would set the gametype to FFA). Cvars can be set with a number of different modifiers. Just typing the cvar defaults to "set":
set: change the cvar value.
seta: set the cvar as having the Archive flag, meaning it gets stored in the configuration.
setu: set the cvar as having the User Info flag, which has no real noted effect.
sets: set the cvar as having the Server Info flag, meaning the cvar is displayed in the Advanced Info section of the server browser.