Ever play 20 Questions? The classic first question is "Is if plant, animal, or mineral?"
In my youth, biology courses were mostly descriptive. Everything was either plant, animal, or mineral but people started noticing that things were more complicated than that. For instance, where they thought that bacteria were single cell plants (they had cell walls like plants) they don't have a nucleus. The genetic material, DNA, was just floating around in the cell.
These guys
are sorta weird to. They look like plants but their cell walls are composed of chitin, the material that forms the shells of insects and crustaceans. So fungi don't quite fit the old scheme of things.
So, now, we have several "kingdoms" of living things. Plants and animals are still there but we also have the protista, single cell eukaryotic (the have cell nuclei) beings. That includes protozoa. And there are the monera that are prokaryotic (they don't have cell nuclei) and include the bacteria. The fungi have their own kingdom because of that cell walls thing.
But there are other schemes floating around. One splits the monera into the eubacteria, the true bacteria, and some weird organisms that like extreme conditions of temperature, chemical environments, pressure, etc., the archaebacteria.
Then there are slime molds that sometimes act like plants and at other times act like animals. Some protozoa lack mitochondria which is another kind of weird.
Things are a bit up in the air right now. Scientist would like to have a nice, simple classification scheme, but it looks like that is not to be
