Combos have both the amplifier and the loudspeaker/s in the same box. These are convenient when small but can be awkward to move around if big.
They are good for home practice as they are easy to set up or stow away and are available in small sizes.
They are great in other situations where a high volume is not required such as Jamming with guitarists, backing acoustic sessions or in some cases where the main sound is via a PA system.
Combos can be as little as 10 watts in power or several hundred. The small ones will have a single 8 or 10 inch speaker. larger ones may have a single 15 or 18 inch speaker or four smaller ones.
Where the amplifier requires an external speaker to create sound it is referred to as a head unit. The speakers normally used are enclosed in boxes and referred to as cabinets or ‘cabs’ the combination of amp and one or more cabs is known as a stack.
One advantage is that a single amp can be used with a huge variety of cabs holding speakers from 10 to 18 inches and in as many as 10 to a unit. Not only does this mean that a wide variety of sounds can be achieved it also means that one amp can be used in many situations by using different cabs or combinations of cabs.
Since the stack consists of a number of smaller components it is easier and lighter to handle than a large combo.
A stack can be used in any scenario but particularly for full band situations where the back line needs to make a big contribution to the overall sound.