Evoke is an ability that is was introduced in the Lorwyn block. It appears on mostly Elementals and Spirits. This ability allows the creature to come into play for a (most likely) smaller mana cost, but once the creature hit play, it's sacrificed. However, most creatures with this ability have another ability that affects the board when it hits play. The ability is played, and the creature is sacrificed.
Here is how the order works;
Creature is played Abilities activates Once Abilities Resolves, Creature is Sacrificed.