With transparent bridging, the bridge monitors the traffic on each segment it is connected to. As traffic is sent on any segment, it builds a table by taking note of the source address of each frame it sees, along with which port it was seen on.
If as it sees the traffic, it compares the destination address against the forwarding tables it has constructed. If the destination address is in the table it is either forwarded to the destination on another segment or not forwarded because it was sourced on teh destination segment.
It's called transparent bridging because all of this occurs without intervention from any other external device (i.e., automatically). If a destination address is noted, and it is not in the forwarding table for any port, the bridge will flood that frame to all other ports (this is different than a broadcast, because the source and destination address remain the same .... a destination broadcast address would be all ones (ff.ff.ff.ff.ff.ff).
A bridge will also forward all broadcasts and multicasts.
A transparent bridge and a switch are essentially identical in function (switches are generally hardware based and operate at wire speed).
With Source Routing, each host sends explorer frames. The explorer frames are forwarded, received, and responded to (back to the originator). Each host builds its own routing table for any other hosts it will communicate with. The frame is sent with a complete path to the destination host (in the Routing Information Field, or "RIF"). The Source-Route bridge's function is pretty much just reading the RIF and forwarding the frame according to the RIF.
There is also Source-Route Transparent bridging ("SRT"), which is basically a marraige of the two above. If a frame is received by the bridge that contains a RIF, it is Source Routed. If the received frame does not have a RIF, then it is handled as if the bridge was a Transparent Bridge.
The above is a brief description. There are many other details for each of the bridging types. IF you search the main Cisco site looking for the Internetworking Guide, it has a pretty good set of examples, complete with diagrams.
Good Luck
Scott
