They're called 'Emoticons'
Your friends are using something we call "emoticons", or "emotional icons". They are symbols which represent a particular facial expression. The best way to interpret them is to imagine them as faces tipped sideways.
So if you look at this one - :), and imagine it rotated 90 degrees, you've got a smiling face, with the colon as the eyes, and the parenthesis as the smile.
Here's a quick "crash course" on internet emotion indicators:
:) = 
:D = 
:( = 
>:( = 
::) = 
:P = 
:-/ = 
8-) = 
:O = 
;D = 
:-[ = 
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