Cells are the basic building blocks of all living things. The human body is composed of trillions of cells. ... Cells have many parts, each with a different function. Some of these parts, called organelles, are specialized structures that perform certain tasks within the cell. Example are plants and animals. The cells of a multicellular organism may share common features and functions. These cells that act as a unit make up a tissue. The fundamental types of tissues in animals are epithelial tissues (or epithelium), nerve tissue, connective tissue, muscle tissue, and vascular tissue.