What is Patina?

Patina a fine coating or film on the surface of certain metals, such as brass.  It is produced from oxidation of the metal over a long period of time or age.  Patina may be seen as a different color, saturation, or texture.  I have many beautiful brass findings that have patina, it adds a distinct character to the item. 

Please add your comments as to you knowledge of patina, or how to achieve patina on brass.