An Evoker is a hostile, humanoid mob found in the Overworld. They are one of five types of Illagers ("ill-willing villagers"), the other four being the Illusioner, the Vindicator, the Pillager, and the Ravager.
An Evoker's noises are sinister, saying "Hrrrm," "Harrurm," and "Hun." They also make a "Wololo" sound when performing a spell.
Evokers are considered illagers, which means that they are 'hostile outcasts from villages'. They look similar to villagers, except their skin is gray in color instead of brown. Evokers are found inside particular rooms of the Woodland Mansion, one at a time. They may be accompanied by 1-3 Vindicators. As of 1.14 - Village and Pillage, evokers can also spawn as wave 4 and 6 of Raids, sometimes riding a Ravager.
Evokers are hostile towards Players, Villagers, and Iron Golems within a 16-block range. One of their attacks summons tiny hostile pixies called Vexes. Their second attack spell summons several clamp-traps from the ground in either a ring or a long line, depending on their target's distance.
These clamp-traps, known as Evocation Fangs, are evoked in either a 20-m line for distant targets or an inner ring of 5 fangs with an outer ring of 8. They deal damage upon snapping shut.
Evokers will flee when attacked if not in the middle of casting a spell.
Evokers can be dangerous when ignored, so you have to prioritize killing them first instead of the vexes they summoned. Any weapon can be useful in dispatching evokers, though swords can be more useful in combat as evokers need to be prioritized first.
These mobs were added to Minecraft in 1.11 - The Exploration Update.
- When not in combat, evokers change any blue sheep they find into red sheep, uttering "Wololo."
- This is a reference to the video game Age of Empires where the priest unit would convert enemy units to their own side while uttering "wololo". 
- While summoning a vex, an evoker emits gray swirls above its head, as if under the effects of a Potion of swiftness.
- The evoker was originally going to wear a white hood similar to that of the illusioner. This can be seen in Meet the Evoker. (This hat model was also going to be used for witches and several villager types)
- Evokers are still present in peaceful mode, but they flee from players and don't attack them.
- In hard difficulty, the last raid wave always has an evoker riding a ravager.