Strigi uses plugins to handle its indices, filetypes, and metadata extraction. Currently the filesystem index can be stored in SQLite 3, Xapian, CLucene and Hyper Estraier. The filetype plugins allow Strigi to get at the text content of non plain text files, such as PDF or office file formats. The metadata extraction plugins can tell Strigi information about files -- for example, the ID3 metadata tags from audio files.
The Strigi distribution includes the main indexing daemon strigidaemon as well as a small collection of clients: xmlindexer, strigiclient, strigicmd, deepfind, and deepgrep. Strigiclient has a GUI and allows you to start and stop the daemon as well as populate and search your indices.