By making pacts with the the legendary Avatars, these heretic mages have unlocked the secrets to the forbidden magic of Vana'diel. Summoner (SMN) is available after completing the quest I Can Hear a Rainbow.
Avatars can be obtained through quests by obtaining the appropriate elemental tuning forks. Elemental Spirits can be obtained either through merchants, or by defeating certain monsters that drop the scrolls (these scrolls can also be bought on the Auction House). Each Avatar/Elemental Spirit has an initial MP cost as well as a perpetual MP cost that is based on the SMN's level.
For more information, please see Summoner Avatar Costs.