Downloading is the transmission of a file from one computer system to another, usually smaller computer system. From the Internet user's point-of-view, to download a file is to request it from another computer (or from a Web page on another computer) and to receive it.
Uploading is transmission in the other direction: from one, usually smaller computer to another computer. From an Internet user's point-of-view, uploading is sending a file to a computer that is set up to receive it. People who share images with others on bulletin board systems (BBS) upload files to the BBS.
The File Transfer Protocol (FTP) is the Internet protocol for downloading and uploading files and a number of special applications can furnish FTP services for you. (However, if you are downloading through a Web page, the FTP request is set up for you by the Web page. You are usually asked where you want the downloaded file placed on your hard disk, and then the downloading transmission takes place.)
When you send an attached file with an e-mail note, this is just an attachment, not a download or an upload. In practice, many people use "download" and "upload" rather indiscriminately so you just have to understand the context. For example, if someone says to you "Download (or upload) such--and-such a file to me by e-mail," they clearly mean "Send it to me as an attachment."
In general, from the ordinary workstation or small computer user's point-of-view, to download is to receive a file and to upload is to send a file.