What is Tracker?
Tracker is a central repository of user information, that provides two big benefits for the user; shared data between applications and information which is relational to other information (for example: mixing contacts with files, locations, activities and etc.).
What does that mean?
Tracker uses 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.