Choosing a guitar amplifier can be influenced by many factors, but looking at it from a purely practical point of view is often a good place to start, so you will at least have the number of features you need. If you are performing live then the output power will be a big factor when choosing your amplifier. If the amplifier is forming part of a back line, then you need not be quite so concerned with absolute power, since the main amplification and power will come from the main PA. Smaller style Guitar amplifiers are obviously easier to move around and take to smaller gigs, so there is some considerable advantage with this arrangement. There are a wide range of power outputs shown in this section and the amplifiers here should be suitable for most needs.
On the smaller end of the scale, there are a number of practice amplifiers. These often include headphone sockets, so you can listen to yourself play without annoying anyone else nearby.
Nearly all guitar amplifiers include some type of overdrive circuit. This is just a way to overdrive the front stages of the amplifier, producing a classic distorted sound. There is either a separate input or a gain switch to achieve this effect. Generally this is included regardless, but many amplifiers also have built-in effects units. This may be a simple reverb, either mechanical or electronic, or a complete digital effects generator offering a wide variety of sound effects; so if you are not planning to use external effects peddles, then built-in effects could be a big advantage.
Most of the guitar amplifiers shown are combo style, where the amplifier and speakers are in one cabinet. For larger output powers there is the option to purchase the amplifier separately with speakers in a separate cabinet.