It is the same as :) or :D. It uses symbols to represent a smile or another emotion. In this case it's laughing or smiling a lot.

XD --> internet slang which means laughing. use ur imagination X (eyes) D (smile)

It's an emotion often used on the web. It means an evil laughter.and it is very common in China,too.

at spanish, god=dios, and X=por as a mathematical simbol!!!

i have the same problem, many time alike a emotion, but, i thought that means alike to say: X=for D=god; and i thougth so, because at spanish, is common to say, "its incredible, XD"