Modern cars use a lot of little electronic bits, most tasked with providing the computer with vital information about what's going into and out of the motor, how the transmission's working and what the chassis is doing. But information in the form of electricity going in doesn't mean much if the computer doesn't have some means of turning electricity back into motion; that's where solenoids and servos come in.
All motors and generators, and many sensors, work on something called the Faraday Principle. Electricity is electrons flowing through or over a conductor. Passing a magnet along the conductor "drags" the electrons, producing electricity; this is how an alternator produces power, by spinning magnets inside of a coil of wire. But the Faraday Principle works the other way, too. If you have a magnet inside of a coil of wire, and then pass electricity through the coil, the electromagnetic field produced by the "field coil" moves the magnet. Thus, you can get electricity from movement, or movement from electricity.
Solenoids are the simplest kind of electromechanical device, and work in basic manner described above. A solenoid consists of a coil of wire -- the actual "solenoid" -- either wrapped around a magnet or placed very close to it. In most cases, the magnet is spring-loaded so that it jumps away from the solenoid coil whenever there's no power going through it. So the solenoid is an on-or-off mechanism, capable of operating very quickly but not capable of stopping in the middle of its cycle at some intermediate stage.
Servos start out as an electric motor, which is functionally similar to an alternator but works in reverse. On the end of the motor shaft you have a very small gear, and it drives a gear about four or five times its size. This "reduction gear" has another very small gear attached to its face, stacked on top of the large gear like the layers on a wedding cake. This smaller gear drives another big gear, which might be another intermediate reduction gear, or might drive the servo output. When you engage the motor, it spins the gear assembly and rotates the output shaft. The gears will continue to move until you cut power to the motor, at which point resistance in the mechanism will halt its movement.
Servos find use wherever a valve or mechanism needs to maintain precise control over the valve position. In practical terms, you'll usually only find servomechanisms at work actuating the idle air control valve, electronic throttle control, intake manifold runner controls, hot-air induction controls, and possibly the turbo waste gate or an electronic fuel pressure regulator. Even then, there are alternatives to all but the IAC valve and electronic throttle control. You'll find them more often in the transmission and power steering system, which need to maintain precise valve orifice sizing. The fast but imprecise solenoid valve works better controlling fluids and electrical flow, functioning as a relay. Fuel injectors are usually solenoid valves, as are most types of electromechanical valves in your fuel system. Starter relays and actuators are both types of solenoids, but control the flow of electricity instead of fluids.
