A 0install application is distributed using a feed file. A feed is an XML file that tells 0install how to run the application, and which versions are available. The feed file is normally published on the web; the URL of the feed is the unique identifier of the application.
To get a 0install application:
Find a program you want to run on the web:
Either launch it directly:
$ 0launch http://repo.roscidus.com/games/openttd
Or generate a shortcut to launch it later:
Upon first launch of a feed, 0install may ask you to accept keys and be allowed to download files.