The Evoker job is one of the jobs bestowed to the party upon resurrecting the Water Crystal.
The Evoker marks the debut of Summon Magic to Final Fantasy and it does so in an interestingly unique way. Summon magic is actually split into three different types: White, Black and High. The Evoker can use White and Black summon magic leading to one of two effects. The catch is that the Evoker has no control over which spell is cast.
Summon Magic is a game of roulette. The Evoker only gets a handful of spells, one for each level essentially. When the Evoker uses a spell, the game determines one of two effects: A White Magic or Black Magic effect. Because the effect is random, making the most out of the Evoker involves determining which summon to use in a particular circumstance.
How the player can approach Evoker varies between versions for the same reason Black Mages require a different approach: In the 3D versions, status and instant-kill abilities are generally far less effective, likely because there are less enemies to begin with. This cripples Shiva and Ramuh as there's a 50% change that the summon will be entirely wasted. For similar reasons, some Evoker summons are significantly more of a gamble against bosses than others.