Gold jewelry comes in several degrees of purity, but it's all real gold. A measure of gold's purity.
Gold used in jewlery is mixed with harder metals for strength and durability.
The higher the percentage of pure gold, the higher the karat. They both look great and wear beautifully. 18k is more valuable because it contains a higher percentage of pure gold. It's like comparing good Chardonnay to fine Champagne. Large pieces in solid gold are heavy and expensive.