What kind of error lights does Xbox 360 have? What are the meanings of each error light?
There's a comprehensive list on this XBox forum, for example:
As you can see two of the error states aren't particularly informative in and of themselves. You have to use the secondary error codes to determine exactly what's wrong.
As well as varying combinations of flashing red lights (known as the Red Ring of Death or RRoD) there are secondary error codes you can obtain from the console - assuming that you can get your console that far.
This Microsoft Support page details the steps to take if you get three red lights (the top right one isn't lit). The problem may be the console itself or the power supply. This page is an interactive guide to determine exactly what the error is from the lights that are flashing.