A guitar amplifier (or guitar amp) is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker and modify the tone by emphasizing or de-emphasizing certain frequencies and adding electronic effects.
The amplifier comprises a preamplifier stage, which amplifies the voltage of the signal from the guitar, and a power amplifier stage which delivers a higher current to the speaker to produce sound. The preamplifier stage may also have electronic effects such as distortion, chorus, or reverb and additional controls such as a graphic equalizer. Amplifiers may use vacuum tubes (in Britain they are called valves), or solid state (transistor) devices, or a mixture of both.
The full stack is the dream of many a guitarist (but will be frowned upon by your soundman and everyone on stage with you). This is usually a 100 watt head connected to two 4x12 cabinets, although other wattages are sometimes employed. The cabinets are stacked vertically (one on top of the other), giving the setup its distinctive name. A full stack is as tall as a grown man, making for quite an impressive sight. The sound is equally impressive. If you set one of these up in your living room and play it to its full capabilities, you will be evicted from the neighborhood (unless you are an isolated hermit). A full stack is too large for all but the very largest of venues, and even then your soundman will be mic'ing you so you'll never actually have a use for a full stack. Most working pros will use two half stacks in stereo rather than bringing a full stack on the road.
There are two configurations of guitar amplifiers: combination ("combo") amplifiers, which include an amplifier and one, two, or four speakers in a wooden cabinet; and the standalone amplifier (often called a "head" or "amp head"), which does not include a speaker. Guitar amplifiers range in price and quality from small, low-powered practice amplifiers, designed for students, which sell for less than US$50, to expensive "boutique" amplifiers which are custom-made for professional musicians and can cost thousands of dollars.