Movement- Wheels, treads or legs?

The simplest robots could be like one armed industrial robots that are fixed in place and indoors and plugged into the wall for continous power and operated by a person.

Of course the most advanced robot would be an ultrarealistic looking and functioning indoor/outdoor autonomous humanoid robot with highly advanced AI and machine learning with a way to recharge itself and right itself if it fell or was knocked over or destroyed (Terminator style).

But for most projects the basic question starts off with what will the robot do and how will it move.

If it is move indoors than you will have a flat level surface and your choices are wide open all the way to animal or human like movement.

But basic moving robots will have either 2 or 4 wheels or perhaps tank like treads that can go over tough terrain.

The older Joe Robot by WowWee has an amazing idea of using a set of 3 spinning wheels so that it can go backwards, forwards, and sideways . This is similar with the crab like RoboQuad by WowWee which was inspired by the design that the creator made for JPL.

Simpler design will be 2 or 4 wheels or even 3.

There is a way to do the one wheel unicycle design which is very complex but involves sensors like gyroscopes to keep the toy or robot balanced.