You can see your status for each faction on your character screen. There are three notable levels of reputation that you can achieve with every faction:
Default reputation level, the Quartermasters will sell you food (which restores more health/mana per second than regular food of the same level range).
At 10000 reputation, you can buy the faction's mediocre items (green-quality).
At 25000 reputation, the Quartermasters will sell you their rarest equipment (blue-quality items or epic and legendary items for certain factions).
League factions, in the order of appearance in the game:
The War Dogs Squadron
Soldiers of Fortune - reputation with this faction is requiered to obtain access to battleground PvP gear