The client will continue to seed the file(s) even after you've downloaded it all. Once the client reports the file(s) being fully downloaded, it's good manners to keep seeding for some duration, preferably at least to 1:1 ratio. Once you no longer want to seed, ctrl-c.
Also, you can abort and continue the downloading later. The client is smart enough to resume properly.
The "virtual" files are in fact the actual files you're going to get. It's just that their content is garbled until the torrent has downloaded fully.
These notes apply to all bittorrent clients universally. Oh, and please do read
the bittorent wikipedia entry.
