in the electronics world refers to any device that is used to adjust the amplitude
of an electronic signal
passed through it. An ideal amplifier would produce at the output a scale of the input signal, i.e.
Output = A*Input
In practice, this is generally not the case, as the materials, designs,power levels and so on generally limit the idealness (and the linearity) of the amplifier.
Linearity is probably the most desirable quality of any sort of audio amplifier. You do not want the sound outputted by your stereo reciever to sound muddled, crackly, wavy, or anything of the sort. One problem is that the various amplification devices available are not themselves completely linear (though they may be on paper). The response of a transistor or a tube may vary over the range of frequencies you are wanting the amplification. This results in frequency dispersion, or distortion in the output signal. Some expensive amplifiers may have its frequency response curves on a plate on the back, or in the user's manual. Some speakers also have their own frequency response curves. You want this to be as flat as possible.
FET amplifiers are, compared to tube amps are somewhat more clinical in their operation, as clipping may happen much sooner than might happen with a tube amp. Tube amplifiers have, by their inherernt nature, different nonlinearities than do FET amps; this might explain why some audiophiles prefer tubes over FETs, or vice-versa. I am still of the opinion that an extremely well-designed FET amplifier will sound just as good as a well-designed tube amp.
Some amps (like guitar amps, for example) are desirable FOR the various distortions they introduce in the audio. Some people prefer this "warmer" or "sweeter" sound. To each their own, I suppose.
Output power is probably more important in RF amplification applications than in audio. FET amplifiers are found in the smaller of all RF applications, as their overall power handling capability is much smaller. The larger transmitters still employ tubes to achieve their tremendously large power output. In areas such as broadcast radio, the typical consumer reciever is made from the crappiest parts available to drive the cost down; this, coupled with the sizable distortions introduced into the audio by the modulation process, the linearity of the RF transmitter stage is not as high a priority.
If you're not a skilled electronics guru, it may be best for you to keep your fingers outside of your amplifier. There are voltages in some of these devices (especially high-power RF amps) that will seriously KILL YOU. And these high voltages can stick around inside these amplifiers for a long time after you unplug them. So be careful boys and girls.