Why a robot's straight-line motion in one space can look curved in the other.
When a motion planner sends a robot arm from one pose to another, it has to decide how to get there — not just where to end up. That choice is the interpolation mode, and there are two fundamentally different ones: interpolate directly in joint space (each joint's own angle), or interpolate in Cartesian space (the end-effector's real-world X/Y/Z position). This is a close cousin of the forward vs. inverse kinematics distinction — in fact, Cartesian-space motion is only possible because inverse kinematics can be solved, repeatedly, along the way.