An End Effector is the part of a robot intended as a tool or attachment.
It may be a simple gripper, or full hand with opposable thumb. It also includes welding torches, soldering irons, cutters, lasers, paint/chemical sprays(or brushes), vacuums, drills, cleaning/buffing wheels or surgical instruments.
They may also also include the business end of a BattleBot - Axes, Spikes, Flippers etc.
Interchangeable end effectors can be an important feature in an adaptable multi-purpose robot. Although you would hope nobody designs a real robot wih such an embarrassing interchange point as Kryten - the sony Aibo did have a memory stick socket under it’s tail.