Thread Groups

[Coret Bob]

I understand the concept of a Thread Group. What I don't know (and can't find in the 
docs) is what happens if you add more Thread Groups to your Test Plan.
- Can you run them seperatly? Or do they always run concurrently? 
- Are multiple Thread Groups only usable if you use the scheduler?
- Do the listeners report on their own Thread Group or do the report on the collective 
Thread Groups? 
- What would be the practical use of multiple Thread Groups? 
 
Bob