The purity of gold is based upon a "karat" system, said to originate from Karob beans (which have a naturally consistent weight) where 24K gold is 100% pure, and all other alloys are based upon percentages related to 24. For example, 14K gold would be 14/24=58.33% gold.
Gold is extremely "malleable". This means you can take a O.5mm diameter pellet and hammer it into a sheet that is over 0.5 square meters, but so thin it will take flight if you blow on it. You may have "eaten" candy or other food that a chef has decorated with a small piece of such gold foil (called gold leaf).
Gold is highly corrosion resistant and its resistance to tarnishing makes it a favored choice for electrical contact applications. Because copper can diffuse into gold creating undesirable inter-metallics, gold plating is typically applied over a "barrier" layer of sulfamate nickel. Gold plating may also be applied over bright nickel to enhance the appearance of the gold deposit. Gold can be plated "soft" or "hard".
Soft gold is typically very pure and registers a reading of between 50 and 90 max on the Knoop hardness scale. Soft gold offers a high level of corrosion/tarn ish resistance. It is an excellent deposit for wire bonding and surface mounting applications.
Hard gold (91-201+ Knoop) is typically employed in applications requiring a combination of wear resistance, corrosion resistance and a consistently low level of electrical contact resistance.