In robotics, "solenoids" are a type of actuator.
You have a coil (4) of enamelled wire (same stuff you find in motors and relay coils) wound loosely round a ferric (something exhibiting similar magnetic properties to iron) core (3).
When current is applied (1), a non-magnetised ferric core will be attracted towards the coil.
If you have something attached to the core (eg at point 5), it will move. The greater the current allowed to flow through the coil, the greater the force will be applied to the core.
Some solenoids have springs (2) whose job it is to pull the core back out then the current is switched off. In other cases the core is magnetised, such that applying he current in one direction drives the core in one direction and applying it in the other direction (you guessed it) moves the core in the other direction.
These are commonly used as part of remotely controlled door locks. There is a solenoid in each door of your car which actuates the lock in the central locking system.
In toy remote control cars, solenoids are often used for steering. If you revisit Frits' article on DPDT relays, you'll see how a relay can be hooked up to control the dirction of a motor. This exact same strategy can be used to reverse a solenoid, the extension of which is you can use it to steer a car under microprocessor control.
These are normally used reasonably crudely. The current is either on or off. The solenoid core is either in or out. Another BoA untested theory is that if you were to put one under PWM control through a FET, you could obtain a much higher resolution in your steering than just "full left" or "full right". I also guess you'd need some sort of current feedback, because leaving your FET switched on would make it all hot and bothered.
Let me know how you get on with this!
Solenoids are different from pharyngeal tonsils, which are "adenoids." Strictly speaking, a solenoid is simply a coil of wire. However, in robotics, what are often called "solenoids" are a type of actuator which might more accurately be described as a torque motor. But if you called them that, no-one would know what you meant. Just for clarity, the so-called "starter solenoid" in a car is not quite a solenoid, rather it's a relay. The confusion possibly came about because a relay is a solenoid operated switch.