BOM stands for Byte Order Mark (also known as ZERO WIDTH NON-BREAKING SPACE) appearing at the beginning of a data stream to define the byte order (whether it is little endian or big endian). BOM is to indicate that the file is Unicode based.

Possible problem caused by Byte Order Mark
In IE6, it will send the dispaly into quirks mode with its presence before “<DOCTYPE …”

How to check the presence of BOM?
View the source code of the page that does not display correctly in a user agent. Or, open the file in the editor which does not support UTF encoding to see the BOM’s signature on the files.

Suggestion
If the user-agent gives problem when you program for multi-lingual websites, check the editor settings to see what format is being used for saving program files. Notepad++ has the option to convert files to UTF and to UTF without BOM. Use the appropriate settings to convert your files.

A character set related tutorial on W3C site can be found here

Byte Order Mark on Wiki

Article on PHP and UTF