The Desktop is the background on your screen when you're using a Macintosh, Microsoft Windows, and similar graphical user interfaces. The idea is that this screen background is sort of like the top of your real desk, and your program windows are all lying on the desktop in a pile. Some programs may refer to their own "desktops." In this case, the desktop is what you see on the screen when the program is running but no document is open.
The Desktop is also commonly called the Finder (especially under System 7). Actually, the Finder is the software that runs the Desktop (among other things). But since you can only access the Finder while at the Desktop and since whenever you are at the Desktop you are in the software program the Finder, the two terms are often used interchangeably. The Desktop is where all your files are kept, all your applications and documents, the System Folder-all of your life on the computer is available here on the Desktop. You come back to the Desktop whenever you want to open another application, install more fonts, see what's on a disk, delete files, organize, etc. It's kind of like "home base."