What is Weak Head Normal Form?

What does Weak Head Normal Form (WHNF) mean? What does Head Normal form (HNF) and Normal Form (NF) mean?

Real World Haskell states:

The familiar seq function evaluates an expression to what we
call head normal form (abbreviated HNF). It stops once it reaches
the outermost constructor (the “head”). This is distinct from normal
form (NF), in which an expression is completely evaluated.

You will also hear Haskell programmers refer to weak head normal
form (WHNF). For normal data, weak head normal form is the same as
head normal form. The difference only arises for functions, and is too
abstruse to concern us here.

I have read a few resources and definitions (Haskell Wiki and Haskell Mail List and Free Dictionary) but I don’t get it. Can someone perhaps give an example or provide a layman definition?

I am guessing it would be similar to:

WHNF = thunk : thunk

HNF = 0 : thunk 

NF = 0 : 1 : 2 : 3 : []

How do seq and ($!) relate to WHNF and HNF?

Update

I am still confused. I know some of the answers say to ignore HNF. From reading the various definitions it seems that there is no difference between regular data in WHNF and HNF. However, it does seem like there is a difference when it comes to a function. If there was no difference, why is seq necessary for foldl'?

Another point of confusion is from the Haskell Wiki, which states that seq reduces to WHNF, and will do nothing to the following example. Then they say that they have to use seq to force the evaluation. Is that not forcing it to HNF?

Common newbie stack overflowing code:

myAverage = uncurry (/) . foldl' (\(acc, len) x -> (acc+x, len+1)) (0,0)

People who understand seq and weak head normal form (whnf) can immediately understand what goes wrong here. (acc+x, len+1) is already in whnf, so the seq (in the definition of foldl'), which reduces a value to whnf, does nothing to this. This code will build up thunks just like the original foldl example, they’ll just be inside a tuple. The solution is just to force the
components of the tuple, e.g.

myAverage = uncurry (/) . foldl' 
          (\(acc, len) x -> acc `seq` len `seq` (acc+x, len+1)) (0,0)

-Haskell Wiki on Stackoverflow