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Unicellular organisms are composed of a single cell. Multicellular organisms are composed of many specialized cells. Specialized cells differ in structure (size, shape...) and function (the role they perform in the organism).
There may be an additional explanation - multicellular organisms rarely are a mass of identical cells, but are almost always filled with cell types and collections of specialized cells called tissues. Multicellular organisms contain only a handful of tissue classes, but it is enough to produce huge diversity.