Well...your answer was not helpfull at all. Of course I tried to look them up before asking here. I never said I was confused about molecules, what I meant is that I cannot precisely determine what tells the diference between a cluster and a molecule. Besides that what i understand when I hear bulk is that we're talking about the internal part of the compound, not dealing with surface. Well, now that I explained myself better maybe someone could see what my problem is and help me.
In my understanding, a cluster is an aggregate of atoms or molecules so that the properties are somehow between these of the single molecules and those of a macroscopic solid (bulk).
But I suppose you know this already.
A similar problem is maybe defining what makes a particle a "nanoparticle".
They have some "special" properties. For example superficial phenomena becomes important. Or magnetic nano particles do not break into domains like bulk ferromagnets.
The specific size may depend on the material and the properties under study.
So clusters may be made from a few atoms up to thousands, maybe more.