The document discusses cells, which are the basic unit of structure in living things. It outlines cell theory, which states that all living things are made of one or more cells, cells are the basic unit of structure and function, and new cells come from preexisting cells. The main parts of a cell are the cell membrane, cytoplasm, and nucleus. Organelles in the cell like mitochondria and the endoplasmic reticulum help carry out life processes.