The Red Ring of Death (RRoD), shown by 3 flashing red LEDs in the top left, bottom left, and bottom right of the RF Module, or Ring of Light indicates that a general hardware failure has occured internally or externally on/in your Xbox 360. By holding the sync button and clicking the eject button, you will get your first of the four digit code, called your secondary "hidden" error code. Click the eject button 3 more times for the rest of the error code. Keep in mind that:
1 light = 1
2 lights = 2
3 lights = 3
4 lights = 0
When you get your error code, look up what it means, for any hope of fixing your console.
The Red Ring of Death is normally caused by a faulty GPU or CPU that have failed due to either the awful cooling system, or the heat spikes allowing the console to stay heated without cooling after the console after the unit has been powered off.
How it occurs and what causes it:
Normally the Red Ring will only occur when your console crashes/freezes/glitches to where the user is required to power off the unit, and on boot up, the Red Ring will begin flashing. On rare occasions, the Red Ring will appear randomly during gameplay, exept without the normal crash/freezing/glitching. The whole screen will go black, while flashing the Red Ring, with the green power icon in the middle flashing, indicating the disc drive is not properly connected or is in motion. This is usually caused by a faulty PSU, or a short in the console. If it's the PSU, best to just turn off your console and leave it all to itself for a while. Common causes of the Red Ring are a Faulty GPU/CPU (not the chips themselves, but the lead-free soulder under the chips that melt easily), the PSU, Bridged soulder joints under the Southbridge/H(ANA) chips, RAM/ROM cracked/bridged soulder joints.
Common error codes:
0001 - Something power related is faulty (Check to see if your PSU light is orange or red when powered on. If it's green, it's most likely the console)
0020, 0021, 0022, 0023 - Bridged/Cracked soulder joints under CPU (chip under disc drive, sometimes caused by Southbridge or H/ANA)
0101, 0102, 0110 - Bridged/Cracked soulder joints under GPU (again, sometimes caused by Southbridge or H/ANA
1022 / E74 - Bridged solder joint under the CPU
1010 / E68 - Hard Drive Cannot be found