The IR signal emitted by the standard household remote is a series of ON and OFF bleeps of the infrared light, the frequency of light used does not vary much, many remotes use a signal of approximately 40KHz. The message is carried in the time the light is on or off. These on and off combinations can be translated into a binary code which in turn can be translated into decimal numbers. These numbers are the device and command codes. First off, the device code in the signal must match the pre-programmed device code in your equipment, and if it does the equipment will follow the instuctions that have been pre-programmed for the command number included in the signal.
There are two primary formats of IR signal. The most common format is known as Pulse Width Method, this is a format where either the ON or OFF time is constant, and the message is carried in the variations of the non-constant time. The second format is known as Bi-Phase or RC-5 and is primarily used by Philips. In this format both the on and off times vary, the light is sampled at a fixed rate and it's the state of the light that decides if a binary ONE or ZERO is present. Usually if the light is ON, it means it's a ONE and if it's off it means it's a ZERO.
This was just a very brief overview. There are two excellend documents about each format, please read them to understand more. Both of these documents refer to the Pronto HEX format, this is the way IR signals are displayed by the Philips Pronto. The Pronto is a universal remote that can be hooked up to a PC and used to load IR signals up to the PC for examination.
Infrared 101 - by me
Pulse Width Method - by Barry Gordon
Bi-Phase Method - by Guy Kuo