The bulk of the documentation is on the live wiki. This includes details about public APIs, architecture illustrations, query examples and much more.
How Can I Use It?
There are two very distinct ways to use Tracker. One is by the command line (which is more for developer's and people experimenting with the client side querying) and the user interfaces. The command line tools are covered further down.
Using the Tracker Preferences application, you can configure what you want to index and what you want to ignore: