An organelle is a functional subunit enclosed by a membrane within a eukaryotic cell. The name "organelle" means little organ and the name comes from the analogy with organs: what an organ is to a body, an organelle is to a cell. So, the organelle is the organ of the cell. There are different types of organelles in cells, such as nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum, chloroplasts, vacuoles, and they all have a different function. For example, the nucleus is the organelle which main function is control of cell activities. Mitochondria are organelles which main function is energy production.