Golgi apparatus, also called 'Golgi Complex' or simply 'Golgi' is the stack of flattened sacs which are bound by a single membrane also known as cisternae. They are important for packaging and transporting the molecules from one compartment/organelle of the cell to another for secretion from the cell.
As already mentioned, Golgi is the stack of flattened sacs. Golgi has four main regions as: cis-golgi, endo-golgi, trans-golgi and median-golgi networks. The vesicles are pinched off from the endoplasmic reticulum containing proteins and other biomolecules; fuses with the networks. The vesicles are gradually modified here and are further moved to other stacks and finally to trans golgi network. Here, they are packaged and are finally sent to their destination or released from golgi and moved to cell membrane for exocytosis.
Golgi is basically involved in modifying and packaging of biomolecules for secretion from the cell by exocytosis.
It is also involved in the transport of lipids around the cell. It acts as a post office where it packages and labels different items which then sends them to different parts of the cell.
The enzymes in the cisternae modifies the proteins by adding a phosphate group (phosphorylation) or carbohydrate chain (glycosylation) to the protein. These modifications sometimes forms the signal sequence which helps in determining the destination.
The golgi also play a role in apoptosis.