Tasks can be broken down into Sub-Tasks, which are the component pieces of work that make up a Task. For the Task to be deemed complete, all of the Sub-Tasks must have been completed.
Sub-Tasks can only be created once the parent Task has been created. They have much of the same attributes as Tasks, albeit with slightly less functionality. They can still be allocated RAG Statuses as they still have the potential to affect the overall outcome of the Project.
Updating the RAGs of Sub-Tasks will cause the RAG of the Task in which they sit to be updated: "parent" Task RAG status works on the same aggregation process as Project and Department RAGs. However, if you update the RAG of a "parent" Task directly, the same RAG will be automatically applied to all of its "children" Sub-Tasks.
When looking at certain Task views, such as the Calendar, Sub-Tasks are not usually shown by default. On each of these views, there is a check box to "Show Sub-Tasks" which must be checked in order to reveal them.