heres a very brief rundown of what i can remember from my uni studies last year about this stuff:
bridges, like you mention connect 2 networks, the bridge knows about what packets are meant for each network, by either IP or MAC addy's (nt sure which), so if a packet goes from network A to network B, the bridge sends it across.

If a packet goes from a PC on network A to another on network A, it stays on network A

Routers can be used to send data to different networks, usually more than 2, the router knows about which networks can be accessed on each LAN interface and send the packets onto the apropriate interface so that they get to the destination. eg:

INF B INF C
______
|Router|

INF A

The router generally knows what LAN's are on INF A, B and C in the Routing table

so if a PC on INF A wants to send something to INF B or INF C, it forwards it onto the appropriate interface on the router itself. I believe this is correct, anyone please correct me if i'm wrong here..

hoipe this is of a bit of help to you. (apologies for the badly drawn grafix)