Jewelry pieces can be plated or coated (covered) with gold and this may be referred to in several ways as gold plating, gold-filled, gold rolled, vermeil gold plating etc.
The main difference with each one of these is the process by which gold is used to plate or coat the item and how thick the gold plating/coating is, which will amongst other things affect how quickly such plating/coating will eventually wear off.
Vermeil plating is one of the best types of gold plating that could be applied on a jewelry piece. It is usually applied on sterling silver jewelry pieces. An 18ct gold vermeil piece should generally be a sterling silver piece plated with 18ct of gold using the vermeil plating process.
18ct of gold is the same as 18K of gold, ct and K may be used interchangeably.