What is a BOM?
In UTF-16 and UTF-32 are the characters defined by 2 or 3 Bytes, sometimes they give the Byte Order Mark with the first three characters in a file.
Some files (like .htaccess) don't work anymore when there is a BOM in the file.
With Notepad++ it is easy to convert files: (see attach).
More technical info you can find here:
http://unicode.org/faq/utf_bom.html