Gold Ore is a mineral obtained mainly underground, though rare surface sightings are possible. It is less common than Iron, Silver and Copper, and is normally found in formations of 5-30 blocks. Gold Ore is the strongest Ore to spawn on the surface layer. It is one tier below Demonite and one tier above Silver.
Since v1.2, there is a chance that your world will have all of its Gold Ore replaced with Platinum Ore during world generation, which will only allow you to craft Platinum Bars instead of Gold Bars. Those two ore types may look different, but their functionality is the same.