Task Tracker stores a fairly arbitrary set of fields about a task.  This is not extensible except by hacking the database and all of the relevant views.

User-visible fields:

Created: the date that the task was created

Creator: the username that created the task

Deadline: when the task is due.

Owner: the username that the task is assigned to

Priority: the priority of the task.  Priorities are hard-coded to [No priority, Low, Medium, High]

Private: Tasks marked with this flag can only be seen by their owners and list owners.

Status: By default, the statuses are 'done' or 'not done'.  This can be changed per-task-list

Title, Text: free text fields

Internal fields:

Parent: the id of the parent task

Live: Deleted tasks are not immediately deleted.  Instead, this flag is set to false.

Sort index: This isn't actually used as an index, but as a sort key.  It is unique among children of a particular parent.

Joins:

Tasks also have children and comments, which are joined in the ordinary fashion.