A simple question, and probably the best place to start.

Let's take a look at the basics of an Atari system:

• Atari 2600 console - processes controller input and cartridge instructions to create video and audio which are output to the TV
• Atari power cable - takes household current (120 Volts, 50-60Hz AC in the US) and transforms it into 9 Volt DC, roughly equivalent to a 9 Volt battery.
• Controllers (joysticks, paddles, keyboard controllers, etc...) - Transform human movement into signals the console can understand.
• RF cable and switch box - Sends the signal from the console to the TV's tuner as if it were a broadcast channel
• Television - The only way to see and hear what the console is producing
• Cartridges - Contain programs that are "run" by the console, which in turn, determine the sights and sounds generated by the Atari.

I said the basics, didn't I?

Click on any of the above links to go further in depth on that component - this was going to be a one-pager, but there's just too much for a single page...