Robotics is a special category of engineering, because it combines the fields of mechanical, electrical, and electronic engineering into an art and science that is tremendously important to modern industry, as well as full of practical applications in everyday life.
For most hobbyists, robotics is easier to study and understand than pure electronics. This is because robots operate mainly on mechanical principles, which the human mind tends to have a better ability to grasp, while electronics usually work invisibly. You can't actually see anything happening in a circuit most of the time, you just have to understand what is going on, while with robots, you can generally see most of the important things that are going on inside the robot.
A robot's fundamental functions can be very broadly categorized into a few basic purposes:
Movement
Detection
Processing
Control
Communicating
This is the only function category that is required; By definition, a robot must be capable of some kind of movement. If it's not able to move, it's not a robot.
In small robots, movement is usually accomplished with electrical motors. Larger robots (such as those used in factories) often use hydromechanical or pneumatic movement systems, but the kind of robots we'll be working with are all small and moved by small electric motors. These motors are ultra-simple: They have two wires, hook the wires up to a power source and the motor starts turning.
Most robots in use today are programmed to sense some kind of environmental circumstances and react to them appropriately. This is achieved through the use of electronic sensors. Conditions which a robot may sense include, but are not limited to: Light, temperature, humidity, movement, obstructions (such as a physical object in the path of a moving robot), radiation, pressure (of air, water, etc.), and contact with another object. Fairly simple and inexpensive electronic sensors are available to detect all of these conditions, and they can be interfaced to the robot to have the robot respond intelligently to changes in its environment.
Many robots today are "smart", having built-in computers that not only perform simple reactions to environmental stimuli (such as turning on a light when it's dark), but are able to process information received. This has more to do with computes than robotics, but if robots are to become more useful, information processing is an important ability they need.
The processing capability of a typical toy robot is usually minimal; Typically, the robot makes some very simple response to a stimulus, such as blinking a light or playing a sound effect. But some robots are full-blown computers, capable of making complex calculations and adapting to their surroundings intelligently.
Virtually all robots have some way for a human operator to control them; At the very least, an on/off switch. Some simple robots, however, are designed simply to keep running and do whatever they do without human input or interaction. For example, you could make a small "scooter" robot out of a motor, some wheels, and a solar panel; The robot could be designed to simply keep running as long as it received light to power it. There would be no need to turn it off, and it would continue running indefinitely, until it either stopped receiving light or failed for some reason.
In larger robot systems, the controls are often directly wired to the robot, but small robots tend to use wireless systems that communicate via radio or infrared signals, like a television remote control.
Many robots are required to send information somewhere. For example, many robots are used as explorers in the fields of mining, war, etc., mainly acting as mobile camera systems so people can see what is happening in a particular location without having to send a human scout into a place where it might be dangerous to go. Obviously, these robots need some way of sending their video signal back to a human operator. Other robots gather other types of information, but they, too, need to get that information to a human for it to be useful.
Exactly how you go about learning about robotics and doing stuff with it depends tremendously on what you want to do with your robots. If you just want to make robots on a hobbyist basis, you can start with a few very basic and cheap parts. If you want to be a professional robot technician, you might have to undergo special training since getting parts for an industrial-grade manufacturing robot and setting one up in your own home might be beyond the realm of the possible for many people. This page will focus mainly on small, hobby-based robotics; I am particularly interested in "smart" robots, the kind that are capable of processing data that relates to their surroundings and performing intelligent functions as appropriate. I am a computer guy, and so my interest in robotics lies mainly in making the field of pure information (which is basically what Computer Science is) manifest itself in the physical world, which is done with robotics. Robots which simply perform a purely physical function are not that interesting to me; They have their place in the world, certainly, but they will not be a primary focus of my studies in robotics.
Projects
For our first project, let's make the simplest possible robot: A robot which simply moves and keeps moving. For this, you only need a few elements:
Because every robot moves, every robot needs a power source to supply energy. You can use solar cells to supply power to the robot, in which case it will keep on running forever as long as it has light, or you can use a battery. Although the idea of using solar cells sounds pretty cool, it's actually much easier to use a battery because solar cells provide very little power, and to create a robot that runs on solar power you'd either need an unusually large solar cell that supplies a lot of power, or an unusually tiny or power-efficient robot. For this reason, I recommend using a simple battery when you're making your first robots. Virtually any normal household battery cell will do fine; AAA, AA, C, or D cells will all work.
For the robot to roll around, it needs a way to create a rotational force, which is scientifically called torque. The basic way to create torque is with a small electric motor. These things are great to work with: They're very quiet, they can be made very small, and they're surprisingly efficient, creating quite a decent rotational speed and force with remarkably little power. They're also very easy to use: They simply have two electrical contacts, and when you connect each contact to opposite sides of a power source, the motor starts whirring around. You can buy small electric motors for hobbyists at most local electronics stores for cheap (a few dollars), and these are usually made to work with 1.5 to 3 volt power sources, which is perfect for use with household batteries. (AAA, AA, C and D batteries all produce 1.5 volts; The difference between them is in how long they last and how many amps they can supply, not in their voltage.) For more information on DC electric motors, click here.
Once you connect the motor to the power source (via the judicious use of a little electrical wire), it'll start turning. At this point, you could say you've made a robot, which is technically true. But a small motor which just keeps turning doesn't really look like much of a robot. To make a really cool 'bot, you need to add some wheels or tracks to give it a way to move around. All you need at this point to make a robot that moves around is some mechanical linkage to convert the turning of the motor into a rolling of the robot. Because this is a purely mechanical task, it is beyond the scope of this page to establish how to do this; There are many ways to hook the motor to a small set of wheels. You could use gears, pulleys, or other mechanisms. This is a time to experiment and try different things to see which ways work best, and which way you prefer to do things. Have fun!