Homosapiens: Humanity collectively is currently the apex predator due to the evolutionary advantages of opposable thumbs allowing us to use tools (grasp things) and superior intellect which allowed us to create those tools in the first place. The main difference between humanity and the rest of the animal kingdom is our use of complex language and communication, animals do have ways of communicating but none of them are as complex and accurate as human language. Humans also have a concept of time, and self-awareness unlike most animals,  and the ability to question our existence is a defining trait -  to be able to domesticate other animals and lead lives not only for survival but for self progression and indulgence (pleasure). Omnivores that can survive off animals or plants.
Animals: Can display intelligence on smaller scales, but are mostly instinctual creatures with a focus primarily on survival, and reproduction. You probably won't see an animal with a tool, unless perhaps you have an incredibly intelligent ape on your hands, but they don't have the same capacity for thought as humanity. They constantly fight to survive, with  many predatory relationships driving the circle of life. Carnivores, omnivores, and herbivores - a wide range of diets there.
Fungi: The decomposers that take care of most of the worlds waste, at least in non-industrialized areas, they live off dead organisms (gross), and despite looking and acting similar to plants are extremely different. The main difference is that they cannot produce their own food.
Plants: Aside from water, plants are the source for all life, without plants fungi starve and die, herbivores starve and die, carnivores starve and die, then humans starve and die. Plants produce their own food using sunlight and soil nutrients. Pretty much the only autonomous and self-sustaining living organism on the planet, that we've found yet. (Wouldn't it be lovely to live off sunlight?) Plants are the basic of the basic necessity for everything following to survive. Plants came before all multi-cellular organisms, disregarding the first bacterias.Â