The Covenant controls the Alliance-exclusive district of the Silver Enclave in Dalaran. The guards they employ are Silver Covenant Guardian Mages, who expel Horde players once they get too close to the Enclave's outer walls.
Before patch 3.1, there was no way to gain reputation with the Silver Covenant. However, now they are the alliance's host of the Argent Tournament and reputation can be gained through many quests. Another way of gaining reputation with them is spillover from the Alliance Vanguard and its associated factions (usually by running heroics without a tabard). You will get an average of 7 rep per mob and 125 per boss. When players reach exalted reputation, they can get a Quel'dorei Steed, Silver Covenant Hippogryph and a Shimmering Wyrmling from the quartermaster, Hiren Loresong.
In patch 3.3.0 the champions of the Silver Covenant will fight the forces of the scourge in the Frozen Halls and its Sunreaver counterpart in Dalaran to restore the magical blade Quel'Delar from its defiled state by the Blood-Queen Lana'thel.
After completing "The Valiant's Challenge", the following quests open up allowing players to repeat the valiant stage of daily quests (steps six and seven) for the other factions with which they have not yet earned the right to champion: