The documentation for our plugin system can be found here.
To start, I recommend you install our hello_world.tar.gz sample, and see how it works, and go from there.
Plugins are quite simple, they're basically just a list of files, extracted to a directory, some of which are scripts to execute code you want. Included in the files are also "hooks", data that is inserted into the DA skins allowing you to add links to your plugin from DA itself.
Other related features with the plugins:
Ability to completely control the socket, which lets you send files. It allows you to remove the DirectAdmin html "bubble" (header and footer) and to also give you control of the headers. Without this option, your output is inserted into the middle of the DA window.
More related items in the versions system.