This article is from the Ethernet FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.
A baseband network has a single channel that is used for
communication between stations. Ethernet specifications which use
BASE in the name refer to baseband networks.
A broadband network is much like cable television, where different
services communicate across different frequencies on the same cable.
Broadband communications would allow a Ethernet network to share the
same physical cable as voice or video services. 10BROAD36 is an
example of broadband networking.