There are 5 levels in the levels of organization. Tissue, organ system, cells, organism and organ.
Tissues are groups of similar cells that perform a common function. Examples are Epithelial, muscle, nervous and connective.
The organ system is groups of organs that work together to perform a certain task. Examples are circulatory, reproductive, digestive, nervous and respiratory.
The cells are basic units of all living things. Examples are Amoeba, Euglena, Paramecium and Volvox.