Part of the the definition of a plant is that it has/makes chlorophyll. They don't, thus they technically are not plants. There are some other differences too, but I had to clear that stuff out of my brain to make room for tango.Does that mean fungus isn't considered "plant"? I guess that makes sense. Come to think of it... there are probably other things that don't fall into one of those 3 categories.
(i.e. I forgot).