The Pi *is* a computer in and of itself. This introduction might help explain some things: http://lifehacker.com/5929913/build-a-xbmc-media-center-with-a-35-raspberry-pi
It sounds to me like the confusion you're having surrounds connecting the two devices over your network. You need to serve the media files from your other computer so that the Pi (and other devices) can access them. You could do this via a straightforward shared folder. We need some more detail about your other computer (and network setup) to help you out further. If you're running windows then this section of the XBMC Wiki should help you out.
Once the files are shared on the network you can configure XBMC to look at that folder as a "Media Source": http://wiki.xbmc.org/index.php?title=Media_Sources
Other options for getting media on the Pi are: USB hard drive directly connected, Network Attached Storage (NAS).