Below are some simple definitions and examples of the different between download and upload.
What Does Download Mean?
To download something is to "bring it down" from the Internet and use it on your computer. It can apply to any remote source like an FTP server, website, or even another computer on your network.
You only download something if you're taking it from elsewhere and giving it to yourself.
Example: I downloaded an MP3 file from YouTube.
In this example, you're downloading the MP3 file because it's coming from a remote server (YouTube) and being received by you.
What Does Upload Mean?
Upload, as it would seem, is the opposite of download. To upload something is to take something from your own network and save it to a remote server like an Internet website, FTP server, etc.
Just like with downloads, uploads are only appropriately called uploads when they involve the transfer of files from your network to a remote one.
Example: I can upload my pictures to Dropbox to back them up. That way you can download them to your computer.
In this example, you'd use upload first because you're moving files from your computer to a remote server - Dropbox. Download is used in the second sentence because it's referring to someone else taking your file from Dropbox and saving it to their computer (like the download example from above).