Glossary
End-effector
The business end of a robot arm — the gripper, hand, suction cup, or specialized tool that interacts with the world.
The end-effector is whatever is attached to the last joint of a robot arm — the part that actually touches the world. Most commonly a parallel-jaw gripper. Sometimes a multi-fingered hand, a suction cup, a welding torch, or a specialized tool.
End-effector design constrains what tasks the robot can do. Parallel-jaw grippers are reliable and ubiquitous but can't manipulate the way a human hand does. Multi-fingered hands are more capable but harder to control. Suction is fast and reliable for flat objects, useless for clothes.
When a paper or model talks about "the action space," it usually means commands to the end-effector — either its 6-DoF pose (3D position + 3D orientation) or velocity. Joint-level control is one level lower in the stack.