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.
The 3D remake keeps the Evoker largely the same as it was before. The most interesting difference, however, is in what didn't make it into the game. Unused dialogue suggests that Luneth was going to gain access to the Evoker in Castle Hein. This would have allowed for an interesting quirk of Summon Magic: It is unaffected by Hein's Barrier Shift. Additionally, an interview with the developers reveals that they did consider giving the player some degree of influence over the summon effects. In the end, they opted to leave it as is.
Summon Magic is a game of roulette. The Evoker only gets a handful of spells, one for each level. 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.
In addition to the standard Knight/Viking, the strategy guide acknowledges that the unreliability of the summon magic means that you'll want more reliable sources of healing and elemental damage. Evokers can also use rods and staves, meaning that they can put their magic stats to other use via item magic.
Of the summons listed, the three singled out are Escape, Hyper and Bahamur. For Escape, the Chocobo has a 50% chance of performing Chocobo Dash ensuring an escape from battle; For comparison, "Run Away" only exceeds that probability on the third attempt (at 60%) and forces a defense penalty. For Hyper, Both of Titan's attacks deal (Earth) damage, guarenteeing an offensive attack. And for Bahamur, Bahamut will either attack the enemy or give an attack boost.