BY DEFAULT, a router will not pass either Broadcast or Multicast. The router has to be configured to do so (IP Helper, for example).
A switch (or bridge, basically the same thing), will flood both types (unless configured to do so).
The difference between a multicast and a broadcast is that a broadcast will be seen by all devices in the broadcast domain, every device that sees the broadcast treats the frame as if it is destined for that machine and passes the frame up the stack. Somewhere up the stack, some process will determine that the packet is or is not of concern to that machine and dispose of it or send it further up the stack. This process wastes processor time if the frame is not of use to the receiving machine.
A multicast is recognized as a specific group address, and unless there is a process looking for that address (or range of multicast addresses), the frames are ignored (or at least ignored much earlier / lower in the stack) ...saving processor time. Also, because a multicast address comes from a specific range of addresses (Class D), they can be managed by switches and routers that are specifically configured to do so. "Management" can be to filter/drop, or to pass the traffic out to ports that have specifically joined a group requesting the traffic ... saving bandwidth on segments that have no participants. Again, by default, routers will block multicast and switches will flood it out to all ports.
That's it in a nutshell.
Scott
