Evoke is a keyword ability that appeared on creature cards in Lorwyn and Morningtide. The keyword ability actually represents two abilities; a static one and a triggered ability. The static ability is what allows you to play the creature via its alternate Evoke cost. This cost can be paid anytime you could play the card with the Evoke ability. If you do choose to pay the alternate Evoke cost then when the evoked creature enters play the other half of the Evoke keyword will trigger. This ability triggers anytime a creature enters the battlefield after having its Evoke cost paid. When this triggered ability resolves, the Evoked creature will be sacrificed. As Evoke creatures all have either triggered abilities that cause them to trigger when they enter the battlefield or leave the battlefield, playing them via the Evoke cost ends up causing these abilities to trigger.