Cabinet speakers are more often known as speaker cabs and consist of a speaker in a box. They are used to get more sound from a guitar combo or deliver sound from an amplifier head.
A speaker cab can have one, two or four speakers in it depending on what you need. The most popular configuration is having an amp head with one or two four speaker cabs. When getting a cab, always ensure that it can handle the amplifier power and that its impedance is suitable for your particular amplifier.