Normal -

We'll be using this as your standard "normal" face. No expression at all... Unless your character is dead inside. If so, then I guess it could mean anything.

Smile -

Just by pulling the mouth up at the corners we can make these two smile.

Laughter -

Its the jaw dropping open that lets out the laughter. As the cheeks pull the mouth open they will also push up the lower eyelid.

Enjoyable Rage -

Mix an open smile with a frown and we get this little gem.

Resentment - 

General displeasure, resentment, or the moment before the character realizes he, or she, is angry.

Anger -

Point the edges of the mouth and the eyebrows downward.

Furious Anger -

A little extra wrinkling in the eyebrows and the gleam of teeth are what form this look.

Fright -

Surprise -

The eyes open wide and so does the mouth. The eyebrows also lift up. Depending on how surprised your character is, you might consider playing with the height of the eyebrows and how far the mouth drops open.

Worry (or Concern) -

The eye brows slope upward. The mouth opens only to whisper a cautious or caring , "Oooo."

Sorrow -

Basically the same facial construction as "Worry," but with tears forming at the corners of the eyes. The mouth might open a bit to form something sounding "Aww-ish".

Crying -

The eyes shut, the mouth opens, and the eyebrows really slope upwards. At this point you might want tears streaming down the cheeks, is which case all you have to do is draw lines down the face.

Embarrassed -

The eyes will shut and the eyebrows slope upward. The mouth opens, with or without teeth showing.

You might want to draw blush lines. I say go for it. They're fun to do.

Thinking -

Usually someone looks into their upper right when thinking hard. The lips are pulled up and to the side. One eyebrow up, one down.

Shyness -

Awe... So cute!

A small smile and a little blush can pull off this look. And don't we all look away when feeling shy? Ha-ha.

Gentle Understanding or
Content Happiness-

Well... I just thought they looked cool. Maybe it also means they're "too cool for school." I guess you could use this look for a lot of things if you wanted to.