*No* expresses a general negative, such as when disagreeing, or indicates an absence of any of a particular noun. 

> No, I do not like coffee.
>
> There was no coffee in his house.

(According to wikipedia): *Not* is the declarative form of no. 

I'm pretty sure it classifies as an adverb, and negates the action of the verb in the emphatic form (do + verb). 

> I did not go to school today.
>
> The train did not arrive.