The task is the central focus of this application (kinda obvious I guess, right?). A task is something you need to do. A task could be something small like taking out the trash, or something bigger like developing a web based application to track tasks.
Larger tasks can be broken up into sub-tasks to be managed more easily. The status (% Complete) of a task that has sub-tasks is based on the status (% Complete) of the sub-tasks and is not editable. Read more on Task Hierarchy.
A task has a number of Properties that you can use to track something you have to do or to look back at something you've already done.