The faces can range from happy to surprised and it is most commonly used by anime fans. All of them exaggerates in facial expressions as most anime shows do.
A brief list of expressions include:
^_^ = Happy
O_O = Shocked or surprised
^o^ = Laughing
@_@ = Confused
-.- = Sad, disappointed or embarrassment
-.-* = Angry
O.o = Raising eyebrow.
oO_Oo = Blushing.
These, are only many of the hundreds of possible expressions you could find and create.
Animefanatic2: Nah, I've seen girls who are much cuter than her.