The difference is obvious. Download speed specifies the data rate at which the data flows into your machine and upload speed specifies the data rate that moves away from your machine.

Usually, both will be equal (Atleast in India internet providers offer same download/upload speed). Usually the two sppeds are measured separately and your home DSL connection will be full duplex. Full duplex means communication is possible both ways and only one side data rate will be specified for spee. For example, if you DSl connection is of 2mbps, you can download at a speed upto 2mbps, at the same time you can also upload at 2mbps. (The BitTorrent p2p file sharing protocol makes best use of this nature of DSL lines). So you effectively get 4mbps (2mps download+2mbps upload ) bandwidth.

Hope that's clear.

Cheers!
-Kalayama