Cell theory states that all living things are composed of cells, cells are the basic unit of structure and function, and new cells are produced from existing cells. Cells can be classified as prokaryotic, which lack organelles and a nucleus, or eukaryotic, which contain organelles and a nucleus. Eukaryotic cells can be single-celled or multi-cellular, and multi-cellular organisms contain specialized cells that perform distinct functions like transport, storage, photosynthesis, and more.