DownloadType: A simple description of a download type.
Downloader: These plugins provider the connection to the downloader programs
Indexer: These plugins provide connection to the sites that have information on releases
Notifier: These plugins provide means to send notification to third party services or other program’s on certain events
Provider: These plugins connect to databases and create elements based on the media type they work for.
PostProcessor: These plugins move rename or do whatever they like with a given path and element.
System: Well there is only one of this kind and maybe there will never be more. The system config is a plugin of this type
Filter: These plugins can filter found downloads as they wish.
MediaAdder: These plugins can add elements. From various sources e.g. Trakt
MediaTypeManager: These plugins are the core of XDM. These plugins define the structure of a media type and provide functions to save and store them. The look is also defined here.