Desktop is a traditional name for a PC associated with a desk position and also includes towers, mini tower and small form factor
Where laptops and tablets are highly portable, a desktop generally comprises of a base unit (usually on or under the desk), a screen, a keyboard, a mouse.
Additionally other devices (printers, speakers, projectors, addition screens, additional drives etc.) can easily be physically (or Wi-Fi) attached.
Upgrades are generally easier as components are often transferable, but it should be remembered that motherboard evolution can often mean that a 'card' that
fitted in your five year old PC may not be compatible with your new one.
A big advantage of desktops over laptops can be the power/price ratio. A desktop will nearly always out perform a laptop of the same price.
in general the following should be considered when purchasing a desktop:
Physical size: will it fit on / under the desk?
Storage size: how big / fast do the drives need to be?
CPU speed: how fast does the processor need to be and how may do I need?
RAM size: how much memory will I need for the PC to perform efficiently?
Ports: what do I need/want to connect to the PC?
The other advantage is that if you don't get the configuration right first time, it is more likely that you can upgrade a desktop later.