Final answer:
An organism consisting of a group of cells is a multicellular organism, composed of specialized cells that form tissues, organs, and organ systems to perform complex functions.
Step-by-step explanation:
An organism that exists as a group of cells is typically referred to as a multicellular organism. These organisms are structured at several levels, with cells combining to form tissues, which then arrange into organs and organ systems. Most visible life, including humans, plants, and many animals, falls into the multicellular category. The cells in multicellular organisms are specialized and work together to perform complex functions, making them distinct from unicellular organisms, which comprise only a single cell.
Biologists classify organisms using taxonomy, which includes multicellular forms ranging from simple clusters without specialized tissues, to complex structures with cells organized into distinct tissues, organs, and systems. Multicellular organisms have various forms, including colonial and coenocytic structures, but what unites them is their cellular organization beyond the single-cell level. They also may display various life forms, such as the caterpillar and butterfly stages of Lepidoptera, or the different forms of cellular slime molds.