If you are reading this then you already have an interest in the robot genre and may well have your own ideas of what constitutes a robot, real or fictional. Be that as it may, I would venture that you would be surprised to discover the scope of the term "robot" so I shall endeavour here to at least give you a frame work on which to hang the information presented in the following pages for, as you will see it is the very starting point of many of the robots you will meet and perhaps build yourself.
There are many learned tomes which discuss the multitude of robot variations and to try to precis them here would be both unrealistic and unhelpful so I will only give the definitions for those that we will meet later.

The simplest definition of a robot could be "a mechanism which moves and reacts to its environment", and there are many robot starter kits which use this definition in some form. The simplest of these I have seen is a fact file type book with a tray of electric and simple mechanical components together with an envelope of card board press out parts and stencils allowing the construction of a variety of wheeled and legged mechanisms which are crudely able to interact with the environment.
Although the build quality is questionable this and other such kits give a good feel for the spirit of the robot concept.

So our first level of definition is a mobile robot, no surprises there as all humans, animals and insects are the typical role models for our well-established robot genre - here are some classifications of Robots:-

1) Tele-robots - Those guided by remote control by a human operator.
2) Telepresence Robots - Similar to Tele-robots but with feed back of video, sound and other data to allow the operator to feel more like they are in the Robot.
3) Static Robots - Such as the widely used arms employed in factories and laboratories worldwide.
4) Mobile robots - Those which need to navigate and perform tasks without human intervention.
5) Autonomous Robots - able to carry out their task without intervention and obtain their power from their environment.
6) Androids - Robots built to mimic humans.
7) An automated factory production line is a robot, not mobile but still has all the features mentioned below - indeed a domestic burglar alarm system comes under the same heading.

(There are more but this is a good start)

For our basic layout of our generalised robot we shall use those systems which correspond to an animal, even a human - though this is by no means definitive and the real robots can have some, all, or few of these features.

The DOME is one of a range of simple robot kits available which exhibit varied behaviours using different sensors and methods of locomotion, wheels, tracks and legs.

The DOME moves in straight lines until it hears a sound, louder than its minimum threshold setting, on which it stops and turns 90 degrees before moving off again. The sound can be a hand clap or the noise of a collision with an obstacle and in addition if the sound is detected while it is stopped and turning then it will continue its forward motion without completing its turn. This allows the DOME to be controlled using hand claps. It also presents a behaviour which enables it to move around a room floor (mostly) without getting stuck. If while travelling between obstacles a loud sound occurs it "takes fright" and changes its course as if avoiding the source of the sound.
The DOME is not only a good example of a basic robot but it also shows how a simple mechanism can exhibit seemingly simple animal-like behaviour.

So - just what is a Robot? -

Lets say "A Robot is a construction which interacts with its environment either under remote control or programmed control, sensing that environment and reacting to it, static or mobile". A clumsy definition to be sure and not even a good one but one which emphasises the variation to be found in robotics today let alone what is likely in the future.