What Is a Robot?
A robot is a machine that senses the world, decides what to do, and acts on the world physically. Take away any one of the three and you get something else: a tool, a remote-controlled toy, or a computer program.
What to notice
Click through the machines and look at which circle is missing. A remote-controlled car can act, but a person does its sensing and thinking, so it is a teleoperated machine rather than a robot. A smart speaker senses and thinks, but it cannot move anything in the world. A toaster acts, but it only follows a timer.
The edges are fuzzy on purpose. A thermostat senses, decides, and switches a heater, so some people call it a very simple robot. A factory arm that replays the same motion with no sensing is still called a robot in industry, because the international standard ISO 8373 defines an industrial robot as a programmable machine with several axes. Definitions are tools, so pick the one that fits the conversation.
The key idea
What makes a robot interesting is the closed loop. It acts, its action changes the world, and it senses that change. Every other concept on this site is one part of that loop.