its a softsampler, ala kontakt/gigasampler/sampletank.
i have it but i dont really use it. its a bit of a chore to load samples into it. it doesnt seem to have much in the way of effects or anything like that. it isnt good for much to me, i would get sampletank2 for the cost of halion.
the one hting halion does do is import emu files, which isnt a regular feature in most softsamplers.