To start with here are some good sites to refer to about IGMP :
Internet Protocol Multicast
Explains IP multicast addressing, Internet Group Management Protocol, how multicast in Layer 2 switching works, defines multicast distribution trees, how multicast forwarding works, protocol-independent multicast, and more.
IP multicast control (IGMP)
Explains that IGMP is used for multicast traffic, the IGMP message structure including query, report, and leave group.
Multicasting
Explains what multicasting is used for, what is needed to use it, and the IGMP protocol used to provide multicasting.
And with above all a cool novice user link for newbies : Protocols