Traditional dial-up modems provide online access through the public telephone network at up to 56,000 bits per second (kbps). A cable modem, on the other hand, provides

high-speed Internet access through a cable television network at more than 5 to 50 million bits per second (mbps).

The term "Cable Modem" is quite new and refers to a modem that operates over the ordinary TV cable of STN. Basically you just connect the Cable Modem to your STN TV cable and the cable TV operator connects a Cable Modem Termination System (CMTS) in his end (the Head-End).

Other terms
A short list of some of the other technical terms and acronyms that you may stumble across in trying to understand the cable modem world.

CATV: Cable TV system. Can be all coaxial or   HFC (Hybrid Fiber Coax) based.

Cable modem (CM): Client device for providing data over a cable TV network.

Channel: A specific frequency and bandwidth combination. Used in this context about TV channels for television services and downstream data for cable modems.

CMTS: Cable Modem Termination System. Central device for connecting the cable TV network to a data network like the internet. Normally placed in the headend of the cable TV system.

CPE: Customer Premises Equipment. Used to describe the PC and/or other equipment, that the customer may want to connect to the cable modem.

DOCSIS: Data Over Cable Service Interface Specification. The dominating cable modem standard. Defines technical specifications for both cable modem and CMTS.

DHCP: Dynamic Host Configuration Protocol. This protocol provides a mechanism for allocating IP addresses dynamically so that addresses can be reused. Often used for managing the IP addresses of all the cable modems in a cable plant and the PC's connected to the cable modems.

Headend: Central distribution point for a CATV system. Video signals are received here from satellites and maybe other sources, frequency converted to the appropriate channels, combined with locally originated signals, and rebroadcast onto the HFC plant. The headend is where the CMTS is normally located.

Downstream: The data flowing from the CMTS to the cable modem

Upstream: The data flowing from the CM to the CMTS.

HFC: Hybrid fiber-coaxial (cable network).

Older CATV systems were provisioned using only coaxial cable. Modern systems use fiber transport from the headend to an optical node located in the neighborhood to reduce system noise. Coaxial cable runs from the node to the subscriber. The fiber plant of STN is delta configuration with all optical node fibers terminating at a headend. The coaxial cable part of the system is generally a trunk-and-branch configuration