Depending on context, the term may be used in several ways as follows.

Any User's Machine
A desktop PC can refer to any user's machine, whether Windows, Mac or Linux or style of case. However, the term "PC" by itself would be more common for this generic usage, and the terms "Windows desktop," "Mac desktop," etc. would be more specific. See desktop computer.

Client Vs. Server
The desktop PC is a client (the user's machine) in contrast to a network server. See desktop computer, client and server.

Stationary Vs. Portable
The desktop PC is a user's computer in a desktop or tower case in contrast to a portable computer (see laptop).

On Top or On the Floor
The desktop PC is a user's computer in a case designed for the top of the desk in contrast to a vertical tower case that stands on the floor. See tower.