Guitar amplifiers, commonly called amps, are a device designed to increase the volume or amplify the sound or signal of a guitar. The signal is an electric current which is carried through the pickups of the guitar then sent through an instrument cable to the preamp where the sound is processed and amplified before it comes out of the speaker.
So, there are three different categories in Guitar amplifiers. Valve, Solid State, and Speaker Boxes. Usually, Beginners start out on a Solid state amplifier, as they are designed to suit those applications more.
A Solid state amplifier is made using a chip to amplify the sound and is more cost effective to produce and purchase. These amplifiers can sound great (the famous JC120 was solid state), but most of the time solid state amplification is used for smaller bedroom amplifiers or for more versatile modelling amplifiers.
A Valve amplifier is a more expensive, yet more popular option. These amplifiers are known for their more natural and warm characteristics and are the most common choice for gigging and touring bands. A valve amplifier amplifies the sound produced by a guitar with Power Tubes/Valves, rather than a chip. These amplifiers have a more natural response and are famous for their warm clean tones, and excellent natural overdrive which is often referred to as break up. This is a natural distortion that only valve amplifiers can create, the lower the wattage the lower volume you will achieve your natural distortion.
Speaker boxes, also known as cabs, are usually paired with a Valve Amplifier Head, as the basis for the sound production. A Head is where all the amplification electronics are housed, which are then transferred to the speaker cabinet with a speaker cable.