There are a few different types of guitar amplifiers that are available for you to choose from. If you're looking for a new amp, you need to first decide what role the amp will need to fill.

The first type of amps that came along were vacuum tube amplifiers. Although the technology of tube amps is fairly old, this style of amp is still very popular with most players. The reason for their popularity is that the tone of these amps is usually very warm and natural compared to the other types or more modern amplifiers. Not only do they sound better, they are usually louder than amps of other types at the same power level. Also, most newer, solid-state amps normally don't require regular maintenance, but tube amps do require new vacuum tubes occasionally. Also, it's beneficial to have a matched set of tubes in the amplifier.

Usually there are two types of tubes in an amp - the preamp section and the power section. Some of the common tubes used in vacuum tube amplifiers are the 12AX7, 6L6, EL34, and several others.

There are many different brands of guitar amps that use tubes -- Fender, Marshall, Soldano, Peavey, Genz-Benz, Mesa Boogie and many others -- especially older and vintage guitar amps. Many of the Fender amps, especially the original amps from the 1960s are in high demand by many players. The amps are still so popular that Fender offers new, reissue amps that closely copy the original amps in style and sound.

Solid state amps were invented to replace tube amps, unfortunatley the warm tones and great sustain of the original tube amps were nearly impossible to duplicate in the newer amplifiers. This is not to say that solid state amps are not as good as tube amps, it just all depends on the application and type of music you are going to be playing. Solid state technology is extremely reliable, but many players have to have at least one tube amp in his/her arsenal. Even with all the effects, synths and modelers, the smooth, warm tone of a tube amp is hard to duplicate.

Some guitarists combine the two types of amps, and use what is called a hybrid amp. This type of amps marries both tube and transistor (solid state) amps into one. Usually there are tubes at one stage of amplification and transistors at the other stage (preamp and power amp). These amps usually offer a vast improvement over a straight solid state amp if you're looking for the warm tones of a tube amp.

Digital amps were introduced in the chain of solid state amps, and offer the guitarist an expanded library of possible sounds that can be had from a single amp. These amps use computers and integrated circuits to create the various tones in the amps. Usually, there are many different types of effects and tones available to the player, with the added versatility of  attempting to duplicate different amps and speaker cabinets combinations. These amps are very versatile.

Examples of some digital amps are Vox Valvetronix, Line6, and a few more. These amps are great if you want to play all different types of music with different types of guitars.

It should also be noted that Vox manufactures a device (amp/cabinet modeler) that offers many different combinations of amplifier and cabinet settings. Although it's considered a modeler and needs to be routed through a speaker cabinet and usually a higher powered amp, the Vox ToneLab offers the guitarist a wide variety of vintage amplifier and speaker cabinet settings. Having a vacuum tube in the device helps it to achieve some awesome tube-like sounds. It even duplicates the sound of a Leslie rotating speaker errily well.