The defining characteristic of multicellular organisms is the possession of more than one cell. In order for a multicellular organism to function, its constituent cells must be capable of identifying and attaching to one another.
Multicellularity gives rise naturally to increasing levels of complexity, which leads to differentiation of cellular functions and the development of specialist cells and organs. Such specialist cells perform vital roles within the organism but are generally not able to survive outside it.
As the number of cells in an organism grows, so too does its size, and so in contrast to most single-celled organisms, multicellular life is frequently macroscopic (visible to the naked eye).
All multicellular organisms are eukaryotic, which means their cells contain membrane-bound structures.
All animals and plants, most fungi, and some algae are multicellular, though most of the Earth's biomass is comprised of single-celled organisms which are predominantly bacteria.