Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus
Fungi (singular fungus) are either uni-cellular, as in yeasts, or multi-cellular, as in mushrooms, toadstools and moulds. Fungi have a nucleus, cytoplasm and a cell wall
While mammalian organisms like us are made up of complex systems and trillions of cells, some organisms are only made up of a single cell! These are called unicellular organisms. Examples include bacteria, some fungi (such as yeast), and certain types of algae. Many unicellular organisms are far too small to be seen with the naked eye, and we use microscopes to be able to see them. There are some examples of unicellular organisms that are large enough to see by eye, such as certain species of algae and bacteria.
See below for an animation exploring the differences in size between animal cells and some unicellular organisms.