From Academic Kids
Multicellular organisms are those organisms containing more than one cell, and having differentiated cells that perform specialized functions. Most life that can be seen with the naked eye is multicellular; all animals (i.e. members of the kingdom Animalia) and plants (i.e. members of the kingdom Plantae) are.
Although some single-celled organisms have differentiated cells (e.g. myxobacteria), the differentiation is less dramatic than that typically found in multicellular organisms. A group of similarly differentiated cells performing a function in a multicellular organism is known as a tissue.
Multicellular organisms must solve the problem of regenerating the whole from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. The overall spatial organization of differentiated cells is a topic of study in anatomy. Multicellular organisms can suffer from cancer when cells fail to regulate their growth within the normal program of development.