To download something means to copy it from some other computer's hard disk to your own computer's hard disk, as illustrated in Figure 1. (By the way, if you're viewing a printed copy of this lesson, be aware that the copy on the screen is animated to show the concept). After you download something, you can access the copy that's stored on your hard disk at any time, even when you're offline.
The term upload means the opposite of download -- to copy something from your own computer's hard disk to some other computer on the Internet. If you can envision the Internet as a "cloud up in the sky", then it's easy to keep track of the difference between the terms upload and download.