To understand the difference between 14K and 18K gold, you should understand the makeup of the metal. 24K is 100% pure gold.  While pure gold is a very strong metal, it is also the most malleable.  The extreme malleability and softness of pure gold make it practically useless in manufacturing jewelry.  In order to be able to use it for jewelry applications, manufacturers mix gold with other metals to harden it.  The difference between 14K and 18K is the amount of pure gold vs. the amount of other metals used:

24K = 100% Gold
18K = 75% Gold and 25% other metal
14K = 58.3% Gold and 41.67% other metal
 
The other metals alloyed (melted, or mixed) with gold will change the color of the final metal, and this is how the color of yellow gold and white gold are achieved.  The ratio of gold to other metals remains the same for yellow and white gold.
 
Should I choose 14K or 18K gold for my wedding band? Below are the differences to help make your decision:
 
1. Scratch Resistance: The 'other metals' are harder than gold.  So because 14K has a larger percentage of other metals than 18K, the 14K ring will be slightly harder and more scratch resistant than the 18K ring.   Therefore, if you lead an active lifestyle, we recommend purchasing 14K gold.  If you plan to wear your rings during activities such as gardening, cooking or athletics, the harder 14K ring would be more beneficial. If you work at a computer or take your rings off before any active use, you would be okay to choose an 18K gold ring.
 
2. Allergic Reactions: Some people are allergic to the metals alloyed with gold. If this is the case, we recommend using Platinum as your wedding band metal, as it is hypo-allergenic. White gold should be your next option as the ring is likely to be plated with Rhodium, a hypo-allergenic metal. If you choose white gold it doesn't matter if you select 14K or 18K, as both will be plated with Rhodium. Yellow gold should be your last option if you are allergic to the metal alloys, as there is no barrier between the metals and your skin.