Version 4.1 - 1.7.a PIM
Protocol Independent Multicast (PIM) is IP routing protocol-independent (as in the name). This means that it doesn't create its own routing tables for multicast forwarding. It uses the unicast routes in the existing routing table (created by any protocol) to perform the RPF check function.
PIM can run in one of two modes - PIM Dense Mode or PIM Sparse Mode. If you look at the underlying assumptions each is based on, it may be easier to remember.
PIM Dense Mode (PIM-DM) makes the assumption that everyone would really like to have this traffic. It floods the traffic everywhere and routers without any downstream listeners prune their branch of the tree. The process of flood and prune repeats every 3 minutes. PIM-DM supports only source trees and cannot be used to build a shared distribution tree. PIM-DM only has (S,G) entries which show the source and group (multicast IP).
PIM Sparse Mode (PIM-SM) makes the assumption that nobody wants the traffic unless they specifically ask for it. PIM-SM uses shared trees and requires the use of a Rendezvous Point (RP). Sources register with the RP and receivers use join messages to indicate they want to receive that traffic.
Cisco's Technology White Paper IP Multicast Technology Overview covers many of the 1.7 (Multicast) topics.
In addition to PIM-DM and PIM-SM, there is also Bidirectional PIM (Bidir-PIM). This was developed for many-to-many communications. The shared trees in PIM-SM are unidirectional. In Bidir-PIM, the IP address of the RP acts as the key to create a loop-free tree and traffic is routed along this bidirectional shared tree. You may want to take a look at the information about setting up the RP. Know that you can manually configure the RP or use Auto-RP. You should also know about RP mapping agents. The RP mapping agent is the router that sends the authoritative discovery packets telling other routers which group-to-RP mapping to use. [Might be important, right?]
Source Specific Multicast (SSM) - Addresses in the 232.0.0.0/8 range are reserved for Source Specific Multicast (SSM). SSM is an extension of the PIM protocol that allows for an efficient data delivery mechanism in one-to-many communications. Once the communications are established, the router closest to the receiver will work with the source and no longer need the RP.
GLOP Addresses - RFC 2770, GLOP Addressing in 233/8, proposes that the 233.0.0.0/8 address range be reserved for statically defined addresses by organizations that already have an AS number reserved. This practice is called GLOP addressing. The AS number of the domain is embedded into the second and third octets of the 233.0.0.0/8 address range.
And I think that's as deep as I need to go for PIM. I added the GLOP addresses in there since it looked like it would be a juicy distractor answer.