A HTML file should obey the rules of writing HTML files, but not always does. A XHTML file must obey the rules of writing HTML files, plus a few others. A XHTML file that obeys the above rules is called a â€œwell-formedâ€ XHTML file. A XHTML file that is â€œwell-formedâ€ and its structure conforms to the rules set by W3C is called a valid XHTML file. W3C is an organization who sets standards for writing XHTML files. Standards are necessary for the browsers on the market to have similar behavior when interpreting the same XHTML file. Validating a file means to check that file to see if it is well formed and valid. To validate your file / web page, please this official link.