The PCB surface ENIG (Electroless Nickel Immersion Gold) is in
common parlance referred to as "Chemical gold" or "Immersion Gold". The ENIG surface
is composed of a chemically deposited nickel layer with a thin layer of Gold, which protects the nickel from oxidation.
The required layer thicknesses are specified in the document IPC-4552: At least 3-6µm nickel and 0.05µm gold.
ENIG has several advantages over conventional (and cheaper) PCB surfaces such as HASL. ENIG provides an excellent surface planarity (particularly helpful for circuit boards with large BGA packages), as well as a good oxidation resistance (shelf life).
More information can be found here: PCB surfaces.
The ENIG process on Wikipedia: