Unicellular organisms are organisms that are made up of one cell only. This is usually the case for bacteria like Ecoli and otherwise known as prokaryotic cell. Multicellular organism are organisms that are made up of more than one cell and there are many such as humans, animals and plants aka eukaryotic cells. There are exceptions such as eukaryotic cells being unicellular such as amoebozoa and excavata. There are also exceptions in which prokaryotic cells can be multicellular such as yeast and amoeba.
