Kinds classify types. Kinds are to types and type-constructors what types are to values.
Ordinary types have kind *. Type constructors have kind P -> Q, where P and Q are kinds. For instance:
Int :: * Maybe :: * -> * Maybe Bool :: * a -> a :: *  :: * -> * (->) :: * -> * -> *
In Haskell 98, * is the only inhabited kind, that is, all values have types of kind *. GHC introduces another inhabited kind, #, for unboxed types.