It's like comparing an apple to a carrot Cherize, or more specifically, the left lane of a freeway and the right lane- if you don't know which is which, you're pretty screwed.
Upload- from your computer to another (ie through the internet or a local area network)
Download- From another computer (internet/local) to your own.
Think of it as you having a big pile of cartons/boxes that have to be passed to a man above you. You have a small pulley system that takes it from the ground, where you are, up to him. You put a box on the pulley, ie a 'packet' (which is a little thingy of data with instructions on how to 'unpack' it once saved) to the man above who takes it, opens it, and uses it. Once he's received enough, he can put together a use for the packets/boxes. That's uploading. Now imagine the image being broken up into four boxes- the man/woman above is Cantr, who receives the picture in four boxes and puts it up on Cantr.
When you download however, it's the opposite, people are giving YOU the packets/boxes. Your computer is unpacking them and reading what the hell to do with them.
That's why it's pretty important. "Hey Cantr, can I download you that file you wanted?" makes no sense, while "Hey Cantr, can I upload you that file you wanted?" makes much more sense and the politcal-correctness junkies and monsters will remain in their caves.