:) This symbol represents a person smiling. Turn your head down to the left. It is supposed to add a happy mood to the statement which it accompanies.

Same for the XD ...look sideways and imagine eyes squeezed shut and a laughing mouth. There's no pre-set "rules" for making these, we only need to recognise them. For another example, tilt your head to the right and see an expression of horror: D:

Scott Fields is absolutely right. That sign is called a emoticon. Coincidentally, the first person to use it is Scott Fahlman. http://en.wikipedia.org/wiki/-)#Creation_of_:-.29_and_:-.28