It's not available for the PC but on the XBOX and PS3 it's a service that you can purchase for a $50 annual fee. It promotes competitions both for the single player and for clans. To get gold clantags you have to be in a clan that has reached lvl 10 through the competitions and special emblems are unlocked for use in game as your clan reaches certain levels. Also as part of the elite subscription you get all of the MW3 DLC releases for free.

It's an attempt by the gaming company, to capitalize on the pre-existing online gaming community's social, and competitive networking.