While there are more than a couple of dozen different mouth positions or shapes made during normal speaking, we do not use all of them for animation.  In "Digital Character Animation" by George Maestri, he points out that phonemes can be boiled down to eight mouth shapes. 

 
Try an experiment with a friend; watch closely while they speak.  It becomes apparent why many of the shapes are hard to define or even pick out.  The ones provided below will make a useful reference or starting point.

With the teeth together or barely apart it becomes the shape for C,D,G,K,N, S,T and Z. Move the teeth further apart and it becomes the shape for the vowel long E. 

M, B, and P 

The normal position for the lips to produce the correct sounds of these consonants. Lips are not pressed tightly nor do they change in thickness.

Vowel U and OOO 

The lips are in a tight, circular shape and may even purse out some to produce the long U sound or the "ooo" sound in words like crude, rude or food.

A and I and Sometimes O 

The mouth is more open and the lip's sides are stretched slightly to the side.  Notice the jaw moves down on this shape. 

F or V  

The bottom lip is tucked under the upper teeth. At regular speed, the lip and teeth touch rapidly and seldom tuck unless the F sound is used in an angry fashion.  
 

La La La 

Somewhat overdone here, this is the shape for the "L" sounds produce in such words as "Look" and "Follow."  The tongue comes up behind the top teeth and is usually visible.

TH  

For the TH sound, in words such as "That" or "They", the tongue slightly protrudes between the teeth or just barely grazes the teeth.

SH 

The sound often heard in a library or in words like "Show" or "Shut."  Teeth are together and the lips sort of curl out just a bit. 

Whistle 

Super tight lip shape for whistling or odd speech patterns tinged with surprise, regret or pleading.

Smile  

Teeth are close together. Eyebrows are relaxed. The eyes are slightly squinted up by the bunching of the flesh in the cheeks. The mouth corners are pulled back.

Open Smile 

Here, the jaw is moved down which widens the eyes somewhat. This is really a prelude to laughing or some other emotional response involving  speaking.

Pouting or Sad 

Notice the pushed up the lower lip and the humped flesh on the chin. Brow is furrowed  and wrinkled creating the sad eyes.

Disgust or Contempt 

Note furrowing of brow. The upper lip is pulled up slightly in a sneer. The bottom lip is pushed up into the top.

 Rage 

This would be an out of control moment during rage due to the eyes being nearly closed.  Directed rage would call for the eyes to be open and focused on a target. The eyebrows are pulled in and furrowed.

Shock or Fear 

The eyebrows are very high, and the chin or jaw is sort of pulled in. Eyes are wide and the mouth will be slightly tightened, teeth could be clenched, or mouth could be wide open.