How Robots Work

"On the most basic level, human beings are made up of five major components:

A body structure
A muscle system to move the body structure
A sensory system that receives information about the body and the surrounding environment
A power source to activate the muscles and sensors
A brain system that processes sensory information and tells the muscles what to do

Of course, we also have some intangible attributes, such as intelligence and morality, but on the sheer physical level, the list above about covers it.

A robot is made up of the very same components. A typical robot has a movable physical structure, a motor of some sort, a sensor system, a power supply and a computer 'brain' that controls all of these elements. Essentially, robots are man-made versions of animal life -- they are machines that replicate human and animal behavior."

This article from Howstuffworks explores the basic concept of robotics and how robots do what they do.