The Golgi apparatus or the Golgi complex is a series of unconnected cup-shaped structures called cisternae which are bound to the membrane. It was first discovered by an Italian physician Camillo Golgi in 1898 and then named after him. Usually, there is only one Golgi complex per cell. The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids synthesized by the cell. It is particularly important in processing of the proteins synthesized for secretion via exocytosis. It is integral in modifying, sorting and packaging of these substances both for export and use within the cell. It is pretty much like a sorting hub of a courier company which organises the shipments and finishes all the pre-requisites of transportation. It is also responsible in adding signal sequences to the proteins which determine their final destination, perhaps like a despatch note on a consignment! Another important role of the Golgi apparatus is the synthesis of proteoglycans which are a major site of carbohydrate synthesis within the extracellular matrix. It is also capable of phosphorylating the molecules and it also has a putative role in Apoptosis' or cell death.
Structurally, the Golgi apparatus is a cisternal network which is unconnected, unlike the endoplasmic reticulum which is. It has five functional regions, the cis-Golgi network, cis-Golgi, medial-Golgi trans-Golgi network and trans-Golgi. Vesicles form the Endoplasmic Reticulum enter the via the Golgi complex vesicular-tubular cluster fused with the cis-Golgi network and subsequently progress through the trans-Golgi network, where they are packaged and sent to their final destination. Each of these regions contains different enzymes which selectively modify the contents depending on where they are destined to reside. The trans face of the trans-Golgi network is from where the vesicles leave the Golgi apparatus. So it is like a port from where the transport-vesicles leave the Golgi complex.
A typical Golgi apparatus has anywhere from 3 to 7 cisternae, but usually around 6. The cisternae carry Golgi enzymes to help or modify the proteins travelling through them to other parts of the cell. They also carry structural proteins important for their own maintenance. Vesicles produced by the Golgi are: membrane delivery vesicles, which deliver proteins and lipids to the plasma membrane of the cell; lysosomal delivery vesicles which deliver digestive enzymes to the Lysosomes; and secretory vesicles which release proteins outside the cell in exocytosis. The approximate width of the Golgi complex is about 2.5 micro meters.
Sources: Principles of Anatomy and Physiology, by Tortora, and other lecture notes