This article is from the Ethernet FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.
A bus topology is a networking architecture that is linear, usually
by using one or more pieces of cable to form a single line, or bus.
The signals sent by one station extend the length of this cable to
be heard by other stations.
A star topology is an architecture that includes a central device or
hub to connect all stations together. Signals sent by a station must
pass through (and are usually regenerated) by these central hubs.
Since the hub sits in the center and all other stations are linked
through the hub, the architecture resembles a star.