A smile can communicate a million different things depending on the context and type of smile. Though a smile is generally accepted to mean the person is happy, there are many more hidden meanings.
Tight Lipped:
The lips are pressed together and forming a smile. It can be short or wide. In both cases the meaning is quite the same. The person can be happy but restraining himself, or not completely showing his feeling or thoughts.
Smirk
This is a funny one, smirking is when someone displays two different emotions on their face. A smirk is a smug, self-satisfied smile. Usually it’s a sign of sarcasm, embarrassment, or cynicism.
Shy Smile
With the head turned slightly downwards and facing away while looking upwards and holding a Tight Lipped smile, this will make the person look more innocent, juvenile, playful and secretive.
Teeth Display
Some people will widely open their mouths and simply display their upper teeth in hopes it appears they are laughing or are playful all the time, it usually works as it makes them appear happier, friendlier and therefore more likable amongst others and easier to approach.