The BIOS performs a power-on self-test (POST) when it starts up the system, to check out the PC and make sure that everything is working properly. Assuming that the BIOS is able to boot the system far enough to gain access to the video subsystem, it will display information about the PC as it boots. It will also use the video system to communicate error messages to the user. In fact, most boot problems are displayed via video error messages, as opposed to audio beep codes.
An error message can be produced by various different parts of the system, depending on how far into the boot process the system gets before it is produced. Most error messages are produced by the system BIOS, as it is responsible for most of the functions of starting the boot. However, other error messages are operating-system-specific.

There are many thousands of individual error messages; some are more common than others because there are only a few different BIOS companies that are used by the majority of PCs out there today. However, since the exact wording of an error message can be changed by the manufacturer of each system or motherboard, there are a lot of variations.
In most cases, the messages are pretty similar to each other; you may see a slightly different wording in your error message than the ones I have listed here, but if the messages look substantially the same, it is probably safe to follow the diagnosis here. "Disk drive failure" and "Diskette drive failure" are what I mean by virtually identical messages. You may want to consult with your owner's manual regarding some unusual messages, or to ensure that your manufacturer means the same thing with their messages that other companies do (there is some strange stuff "out there").

Please note that similar error messages, those that really mean the same thing but just use different wording, have their descriptions and diagnoses grouped together to make things simpler. There is not a one-to-one correspondence between the error messages listed in the index frame on the left, and the diagnoses shown below. It is best to use the index frame to select the error message you are seeing.