A radio signal is a carrier signal -- a pure sine wave of some specific frequency -- modulated in either frequency or in amplitude by an audio-frequency signal.
This modulated carrier is transmitted from one antenna on a tower to another on your car. The result is that an identical modulated radio-frequency signal is induced in the antenna.
The first stage of radio reception is a radio-frequency filter, usually made with an RLC with a variable capacitance or inductance. This circuit is a bandpass filter, and admits frequencies only in the vicinity of the desired carrier frequency. The vicinity of a specific carrier frequency is called a channel. This filter does not need to be very good -- it just needs to prevent power in channels far away from the desired chanel from reaching the next stage of tuning, the mixer.
In most radios, the filtered radio-frequency channel is passed through a mixer (called a heterodyne in the old days). The mixer is really the heart and soul of the tuner. The mixer sums the incoming signal with a pure tone (sine wave) from a local oscillator with an adjustable frequency. The interference of the two waves produces beats. The beat frequency is much lower than the radio-frequency carrier, and is easier for the subsequent electronics to handle. The beat frequency is sometimes called an intermediate frequency, or IF. The process of converting from RF (radio frequency) to IF (intermediate frequency) is sometimes called "mixing down to IF."
The IF is specific to your radio -- it was selected by the engineers who made the radio. The mixer is followed by some sharp IF filters that select out just one channel. Since the IF filter never changes frequencies, it's economical to build it to be very sharp and very high quality. It would be cost-prohibitive to build a sharp, yet variable filter.
Finally, this IF is passed through a demodulator, which recovers the original audio signal. The audio signal is then amplified and passed through your speakers.
When you tune your radio, you are tuning both the local oscillator in the mixer and the rough RF filter. The mixer's job is to precisely tune a particular frequency. The mixer is not perfect, however, in that it always mixes TWO channels to IF, not just one. The RF filter's job is just to prevent the undesired one from getting through to the mixer.
There's a lot to understand, and I'm sure I breezed through it pretty quickly. Here's an excellent page with more information:
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part16/page1.html
- Warren
