This number tells you the purity of the gold.

• 10k gold is 41.7% pure gold (58.3% being a mixture of other alloys).
• 14k gold is 58.3% pure gold (41.7% being a mixture of other alloys).
• 18k gold is 75% pure gold (25% being a mixture of other alloys).

For a few reasons, 14k gold is slightly more durable than 18k, making it the better choice for those with an active lifestyle. However 18k gold goes retain a more vibrant color than 14k typically does.