The Golgi Apparatus (a.k.a the Golgi Body and Golgi Complex) is an 'oganelle' (small component) found in almost all cells that have a neucleus (i.e. eukaryotic cells).
It packages 'vesicles' (i.e. small pacages of protein wrapped in a fatty membrane) and dispatches them to the correct destination within the cell (i.e ourside the nucleus). It is a fundamentally essential part of the 'machinery' of the cell, and therefore of all complex organisms.
It was first discovered in 1897, though its functions were only identified, in stages, during the course of the 20th century.
Despite more than a century of research, the molecular mechanisms which it uses to sort, package and transport the proteins in unknown. There are currently 5 major models which attempt an explanation.
Though there are multiple models that attempt to explain vesicular traffic throughout the Golgi, no individual model can independently explain all observations of the Golgi apparatus.