Guitar amplifiers are usually either a combo unit or individual units cabled together. The separate amplifier is called an amplifier head and is commonly placed on top of one or more loudspeaker enclosures; the amplifier head and loudspeaker enclosures together form an amplifier stack. An amplifier stack consisting of a head and one cabinet is commonly called a half stack, while a head and two cabinets is referred to as a full stack. The image on the left is of a Marshall guitar half stack, the head is 100 Watts and the speaker cabinet has four 12" speakers. On the right is a combo guitar amplifier image of a Fender FM212R, with 100W and two 12" speakers.
 
While most amplifiers that are used with electric guitars and basses are solid state, some prefer the sound of vacuum tubes. Some modern amps use a mixture of both technologies, with 1960s vintage vacuum tubes next to integrated circuits. With the advent of microprocessors, there have been new "modeling" amps that don't use vacuum tubes and can simulate a variety of vintage amps.
 
Some Guitar Amplifier Brands: Acoustic , AER , Alembic preamplifiers and filters , Alesis , Ampeg , Ashdown Engineering , Bad Cat , Behringer , Bogner , Carvin A&I , Cordford , Crate , Electrovoice , Engl , Fender , Framus , Hartke , Hiwatt , Hoffman , Hughes & Kettner , Ibanez , Korg , Laney Amplification , Line 6 , Marshall , Matchless , Mesa/Boogie , Orange , Peavey , Randall Amplifiers , Rivera , Roland , Ross , Soldano , Sovtek, SWR , Tech 21 , Tone King , Trace Elliot , Vox , Yamaha  

EXAMPLE COMPARISON OF GUITAR AMPLIFIERS, which are shown in the COMPARISONS (along with manufacturer's product descriptions and technical specifications; the prices listed are just examples from one retail source),  features guitar amps in two basic price ranges: around $300 (50 to 100W and 10" and 12" speakers), and around $150 (15 to 25W and 8" to 10" speakers). Of these particular brands, Fender, for example, is known for their Country, Blues and Jazz guitar amplifiers, Marshall is known for heavy metal, Randall for Rock & Hard Rock, and Line6, one of the new modeling amplifier lines, is probably the most versatile for a variety of modern trends. Also guitarist have tastes and preferences when it comes to what sound one is going for. One person will prefer one speaker's sound over another, or require a particular effect or feature that another guitarist might hardly use. Features for recording purposes is not part of these comparisons.

Some say not to get the small 'practice' amplifiers to learn the electric guitar with, because it won't make you sound like that guitar on your favorite CD and one might get discouraged, or will learn so fast you'll be needing a bigger amp any way. This is both true and not the whole picture. Even though a guitar student will outgrow a smaller, less expensive, amp quickly and want something with better, bigger sound and more effects, these small amps come in handy for taking to class and other places to practice and play with friends, rather than always having to haul around that heavy, expensive, and awesome amp you bought.

Some small amplifiers are even battery operated, like the infamous little Pignose. You can take them where there isn't any electrical plug and jam out.  Sometimes price is a factor and a smaller size amplifier can be found that includes special effects and features. A 'small' amp to carry to and from class might be considered as having in addition to a larger, more powerful amp with choice effects and features for home, concerts, and other venues.
  
Typically, a "practice" amp is considered to have ratings of 20 Watts or less, and "performance" amps as generally 50 Watts or higher. One might say, however, that under 15W isn't much to work with and up to 50W are more or less 'practice amps', 50 to 100W are 'rehearsal' amps' for practice, casual playing/small band, or small venue amps and 150W might begin basic performance/stage level equipment.

10" speakers are currently very popular even for bass amps...12" was formerly widely used for bass amplification speakers, and is the speaker of choice for keyboard amps because of the lower and broader range; many guitarists use the beef of 12" speakers and some bassists prefer 10" for pop and funk and some jazz because they pick up the higher ranges where a lot of bass fretwork is done. Bass amplifier combo units are usually heavier, thicker cabinets due to the vibration, and having wheels are handy unless small/light enough to carry.

Useful Features in amplifier heads: although 'basic' amps are just that, volume control, gain, and usually a 3 band equalizer (low, medium, and high), there are other features that are useful to have:

headphone jack (some mute speaker output when headphone are plugged in, some don't)
direct line out (the amplifier head can be connected to other units, speakers, or interface with recording equipment, 'direct recording', compared to using a microphone to mic the amp speaker). An LXR line out is for a microphone cable
auxiliary line in allows connecting to a CD player or drum machine, to play along with
built-in basic effects, such as distortion (overdrive 'fuzz') and reverb (mimics natural acoustics of, for example, sound reflecting off the walls of a small room or a large hall, etc.)
DFX are digital effects, such as delay, chorus, flange, and many other 'special' effects, such as harmonization, multiple kinds of preprogrammed distortion and reverbs, multiple kinds of delay, choruses, flanging, and combinations of these creating different voicings
effects loop allows interfacing other devices such as effects processors...as opposed to or in addition to built-in effects
modeling allows changing the sound of the amplifier to match different amplifiers, such as tube amplifiers and other amplifier and speaker brand signature sounds
tuners are sometimes built-in and some allow for monitoring string tuning real-time

Speakers: small 'practice' amplifier combo units typically have 4" to 8" speakers, 5" and 6 1/2" are common; 10" , 12" and 15" speakers are standards.

A speaker, is an electromechanical transducer which converts an electrical signal into sound. The term loudspeaker is used to refer to both the device itself, and a complete system consisting of one or more loudspeaker drivers (as the individual units are often called) in an enclosure. The loudspeaker is the most variable element in an audio system, and is responsible for marked audible differences between systems.

Speaker types

Woofers: A woofer is a loudspeaker capable of reproducing the bass frequencies. The frequency range varies widely according to design and hence while some woofers can cover the audio band from 50Hz to 3kHz, yet others may only work up to 1kHz.

Mid-ranges: A mid-range loudspeaker, also known as a squawker is designed to cover the middle of the audio spectrum, typically from about 200Hz to about 4-5kHz. The distinction between woofers and mid-ranges is blurred however since many woofers can operate up to 3kHz. These are used when the bass driver (or woofer) is incapable of covering the mid audio range. Mid-ranges typically appear where large (>16cm or 8") woofers are used for the bass end of the audio spectrum.

Tweeters: A tweeter is a speaker which is capable of reproducing the higher end of the audio spectrum, usually from about 1kHz to 20 or perhaps 35kHz.

Full-ranges: A full-range speaker is designed to have as wide a frequency response as possible. These employ an additional cone called a whizzer to extend the high frequency response. A whizzer is a small, light cone attached to the woofer's apex around the dust cap.

In all, comparisons are informative. Music stores are designed for test driving guitars and amplifiers.