Hallucinations are given by external "help". I mean likedrugs, medicines or some kinds of mushrooms; they confuse the brain, which stops working as usual and starts "seeing" things that are not real or can't feel the reality as we know it every day.
For the illusions instead, we don't need any medicine or drug. We have illusions when pur body is under a very big stress (extreme depression and/or stress, mental illness, body stress). Body stress illusions can be easily explained: we all know from when we were young that a person that stays in the desert, without water and under a hot sun will start seeing oasis in the middle of the desert! That's it, an easy example.