Use categories to define the type of a task and to link a workflow to a tasks.
Categories can be thought of as “task types”. Categories define task types by creating a link between tasks in a particular area of the hierarchy, and workflows. When creating a category, you can specify its possible subcategories and the user groups who can create, view and delete tasks of that type. For example, you can specify that only a manager can create tasks of the project type, or that a bug cannot have a project as a subtask. The most basic pre-defined category is the Folder. The workflow for a Folder is simple: it has one state, no transitions, and no custom fields. The purpose of the Folder is simply to allow you to group tasks together, and to define security rules for accessing those subtasks.