Cable Modem Overview

A cable modem is an external device which connects to your computer and is designed to work over cable TV lines.  Cable modems translate radio frequency (RF) signals to and from the cable plant into Internet Protocol (IP); the communications protocol spoken by all computers connected to the Internet. A modem can be connected to a PC using and an Ethernet or USB connection.

Cable modems have several lights on the front that indicate the modem's activity status.  The lights can vary on different brands of modems.  

As a general rule:

Power The power light should always be steady.
Receive The receive light should be steady when downstream signal is confirmed.
Send The send light should be steady when the upstream signal is confirmed.
Online The online light should be steady and indicates the downstream and upstream signals are confirmed and searching for an IP address.
PC Activity The PC activity light will be steady when it is connected to a CPE device and will blink when sending or receiving data.
Standby This will be steady when the modem is in Standby.