Illagers, short for "ill-willing villagers", are humanoid Mobs that look similar to Villagers, except they are hostile toward players, much like the Witch. They are considered outcasts from villages. In addition to this, they will also attack Villagers, and Golems. There are 3 types: the Evoker, Vindicator, and Illusioner.
Evokers and VIndicators spawn in Woodland mansions, while the Illusioner is unused and can only be summoned by commands.
All illager sounds are slightly different from each other, but sound a lot like Villager noises in a "humming" manner. This also applies to Witches.