hi!
For a multicast network, you need both Igmp as well as a multicast routing protocol ( PIM/DVMRP etc.)
The routing protocol works between the routers to send the multicast stream from the source router to the end router. This is the work of PIM ( note that PIM requires any unicast igp for its working).
Igmp comes into play for the leafs where it actually conducts the information of the host's intntions to join a multicast stream to the end router.
So in summary, PIM works in the core while igmp at the leafs.
The following link will give you more details on the matter :
www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.1/19ew/configuration/guide/mcastmls.pdf
Cheers!
Niranjan.