Uploading is when a user copies a file from his computer to another location, and downloading is when a user copies a file to his computer from another source. When one user uploads a file to a network, other users with access to the network are then able to download that same file.
It is a common misconception that a file which is downloaded from one computer is transferred to the new computer, leaving no copy of the file on the original computer. This is not the case. When files are uploaded or downloaded, they are only copied from one computer to another. Some processes might automatically delete a file in order to give the illusion of a physical transfer, but it is always just copying and then deleting.
When users copy music files from their computers to a network, such as was originally done with the service called Napster, this is an example of uploading. However, the terms “upload” and "download” are used more in reference to a client and a server than in peer-to-peer systems, since usually the word “transfer” is used instead. Another example of uploading is when a Web publisher uploads one of its files to a Web server. As a general rule, upload speeds tend to be considerably slower than download speeds.