It's a face.

Raising eyebrow peculiarly: o_O

Blank stare: ...O_O...

Happy: ^_^

Winking: -_^ (even though that really doesn't make sense, because then BOTH eyes are closed, when you think about it, but -_O doesn't exactly look right either, now does it)

Annoyed: _ (Alt+172)

Frustrated: >_<

Shocked: @_@

Bored: -_-

Dead: x_x

Some people use a . in the place of a _ (o.O, ...O.O..., ^.^, etc.), but that's just too cute for me.

Any questions?