The Info.plist file is (usually) the means through which a Quicksilver plug-in lets Quicksilver know what functionality it provides. If a plug-in provides an action, there will most likely be mention of this in its Info.plist file. If a plug-in provides a preference pane (e.g. the iTunes plug-in), it's a good bet that there will be something of the sort in the Info.plist file.

If you're curious about what functionality an existing plug-in provides, you can take a look at its Info.plist file for hints. In fact, it's probably a good idea to spend some time examining Info.plist files for existing plug-ins before you try to put one of your own together. The Info.plist file for a plug-in lives within the Contents directory of a plug-in. So, for example, if you have the iTunes plug-in installed, you might find its Info.plist file at:

Info.plist files are XML files and if you have the Mac OS X Developer Tools installed, opening an Info.plist file is likely to invoke Property List Editor. Most of the relevant portions of plug-in Info.plist files have some explanation available at:

The author suggests choosing a few plug-ins you are somewhat familiar with and trying to get some understanding of their corresponding Info.plist files. If the first few examples you choose appear to be too complex, the author recommends starting with simpler ones and as your understanding increases (or perhaps not) move on to more complicated ones. There are quite a few plug-ins to choose from. One possible place to start looking for candidate plug-ins is on your own machine under:

You can also install additional plug-ins to get a look at their contained Info.plist files.

For more information on Info.plist files, see References and Resources.