Uploading is the transmission of a file from one computer system to another, usually larger computer system. From a network user's point-of-view, to upload a file is to send it to another computer that is set up to receive it. People who share images with others on bulletin board services ( BBS ) upload files to the BBS.
Transmission in the other direction is downloading -- from one, usually larger computer to another, usually smaller computer. From an Internet user's point-of-view, downloading is receiving a file from another computer.
The File Transfer Protocol ( FTP ) is the Internet facility for downloading and uploading files. (If you are uploading a file to another site, you must usually have permission in advance to access the site and the directory where the file is to be placed.)
When you send or receive an attached file with an e-mail note, this is just an attachment, not a download or an upload. However, in practice, many people use "upload" to mean "send" and "download" to mean receive. The term is used loosely in practice and if someone says to you "Download (or upload) such--and-such a file to me" via e-mail, they simply mean "Send it to me."
In short, from the ordinary workstation or small computer user's point-of-view, to upload is to send a file and to download is to receive a file.