XOXO

XOXO represents the typical signature to one's beloved, meaning hugs and kisses.

But how did the X come to mean "kiss"?  Back in the Middle Ages, people who couldn't write and yet had to sign their names on formal documents.  The X is the first letter for the word Christos (Greek version of Christ: Χριστός), so people would sign the holy X and kiss it, thereby sealing their oath to show their sincerity.  (Just as some people swear on the Bible to show they're not going to lie.)

The O is more modern and is speculated to represent the person encircling his/her loved one in his/her arms.

Happy Valentine's day!

Sources:
Kissing facts
Intense research article on the matter