- Cells are the basic structural and functional units of the human body. There are many different types of cells including muscle, nerve, blood cells etc. - Tissues are groups of cells that perform specific functions. The basic types of tissues include epithelial, muscle, nervous, and connective tissues. - Organs consist of two or more tissues working together to perform a particular function like the heart, liver, stomach. Systems are associations of organs that work together like the digestive, nervous, circulatory and respiratory systems.