If downloading is receiving a file from another computer, uploading is the exact opposite. It is sending a file from your computer to another one. Pretty straight forward. It is possible to both upload and download at the same time, but it may cause slower transfer speeds if you have a low bandwidth connection. Because most files are located on huge ftp servers, people generally do a lot more downloading than uploading.
UPLOAD
Meaning of UPLOAD in English
English glossary of computer and Internet terms. Английский глоссарий компьютерных и Интернет терминов. 2012