Data Warehousing clarifications

I have few questions on Data Warehousing as am new to DWH.Following are my questions:
What is business key in DWH. As per i under stood : A PK of a table from which dimension table is loaded. Am I right ? if wrong can u plz explain.
What is the relation between fact table and dimension table.
what is the relation between factless fact and dimension table.
Can you please send me one simple model where it inculdes fact table,factless fact and dimension table.
Is factless fact table a bridge table?
Why operational reports are generated at the ODS why not at the DWH. What happens if its operational reports are pulled from DWH.
In what situation mamy-to-many relationship existing between fact and dimension.
It could be helpfull if you answer my question. Waiting for your reply as early as possible.