Direct realism is the idea that seeing an object O is a way of grasping O that requires no mental intermediary.  Jonathan is right to say that DR entails the non-existence of sense-data, but it also entails that mental states are not representations of reality. 

According to direct realists, there is no specific common element shared by

    (M) the mental state of seeing an object O and
    (Hal M) The hallucinatory state of its merely seeming to one that one is seeing O (where O does not exist). 

The (M) is not equal to (Hal M) thesis goes against the flow of millennia of philosophical thought

DR rests on the thought that (M) is a direct grasping of O, while (Hal M) is not (there being nothing to grasp in the case of (Hal M)).  (Of course, there may be something in common between an (M) state and a (Hal M) state, namely that in both, it looks to the subject as if O is present.  But the claim is that there is nothing specific in common.) 

DR does not allow that, even as far as the mental side is concerned, veridical seeing and hallucinations are specifically the same.

One popular version of DR was propounded by Paul Snowdon, on the grounds that seeing O supports "demonstrative thoughts" while a hallucination of O does not.  Michael Martin is another direct realist.

DR is not the majority view in philosophy (though realism is).  (Jonathan, the fact that both Snowdon and Martin are at UCL may have given you the impression that most philosophers are in this camp.)  As a matter of pure description, most direct realists live in England or were born there.