Yep, Xmas is just an abbreviation of Christmas.
Another explanation is that X also symbolizes the Cross, but since there isn't such a letter as a "+" in the English alphabet, it is most commonly replaced with an X. And being that the Cross is a symbol of Christianity, then it is deemed appropriate that Christmas becomes "Xmas".
Other examples would include road signs that read "Caution : Deer X-ing" which means Deer Crossing.
Anyway, does language matter? I'm a Muslim myself, but I feel that it is the probably the spirit of Xmas that counts, and not what we call it. Xmas is called different names in different languages, does that mean that it denotes different meanings? No, it doesn't.
Click this link if you don't believe me :
http://www.santas.net/howmerrychristmasissaid.htmOne should not turn a non-issue into an issue.
Mele Kalikimaka ame Hauoli Makahiki Hou! (That's Hawaiian for Merry Xmas)