Files can be compressed into one file such as a .zip or .rar file.
This is particularly handy for sending lots of photos - it is easier to attach one zipped file than attaching 50 one after the other.
Uploading files for sharing is another matter. Fine if it's your own material but if it's copyrighted then it's not allowed. Some publishers create PDF versions of their paper books so they can be purchased and downloaded instantly. Illegal uploads could be files like this or scanned documents converted to a PDF file. Torrents aren't actually uploaded but linked to the original file on your computer. Say 2 people get it from you, 4 people might get it from them, 8, 16, 32 etc etc.