What is Tracker?
Tracker is a search engine and that allows the user to find their data as fast as possible. Users can search for their files and search for content in their files too.
What does that mean?
Tracker is a central repository of user information, that provides two big benefits for the desktop; shared data between applications and information which is relational to other information (for example: mixing contacts with files, locations, activities and etc.).
This central repository works with a well defined data model that applications can rely on to store and recover their information. That data model is defined using a semantic web artifact called ontology. An ontology defines the relationships between the information stored in the repository.
An EU-funded project called Nepomuk was started to define some of the core ontologies to be modelled on the Desktop. Tracker uses this to define the data's relationships in a database.