A Bridge vs. a Router
In general, routers pass higher-level networking traffic (IP/IPX/AppleTalk and above --
on the OSI 7 layer model). Bridges carry lower-level protocols, including low-level
broadcasts and routing queries. In practice, many modern models are hybrids (sometimes
called "brouters") and can be configured to handle mixes of traffic levels.
Routing is usually a slower operation as more intelligence is required; routers usually
interpret the packets traveling over the wire and only pass those packets which the
device on the other end of their router-link might need. Bridges on the other hand usually
just pass all traffic coming in to all the other (outbound) interfaces; since there is no
lookups or interpretations they can do this much faster, often at full interface speed (or
close to it).