[nominolo]

A closure is a function + environment, i.e., the values of its free variables at the time of creation.

A thunk is a suspended computation together with an environment. That is, a thunk is a special type of closure, where the function takes no arguments. Additionally, a thunk is updated with its result.

In GHC's execution model, the Spineless Tagless G-machine (STG), every heap object is actually a closure. That is, every heap object has a code pointer that can be called to return the object's value. For evaluated objects (including functions) that is essentially the identity function, for thunks this will evaluate as much as necessary, return, and possibly update it's heap object with its new value. Of course, there are several optimisations going on, but that's the basic idea.

So, I'd say a thunk is a special kind of closure.