Baseband can be thought of as the opposite of Broadband, which everyone who has high-speed internet access should know about.

Baseband allows a single signal on a cable or similar medium. Contrary to popular belief, networking via Ethernet is a baseband process - only one computer can talk at one time. If two attempt to talk (called a data collision), they will stop and wait a random amount of time before retrying.

Cable modems or DSL connections allow multiple signals on the line, usually separated by different frequencies. When connected to your computer, however, the USB or Ethernet cable permits only baseband access. Luckily, there's usually only two or so devices on this short network (the modem and a computer or two). The modem does the work of placing the data from the broadband end to the baseband port, and vice-versa.

Ethernet is digital information sent by an on/off direct current toggle, which is why there is no associated frequency. Two devices trying to send a DC signal would make the data unusable, since their digital pulses would only run into each other. This is why it is considered a baseband medium.