Full-grain leather takes the entire grain of hide, with all the imperfections and inherent toughness of the material.
It's often used for heavy-duty leather items, like weapon holsters and utility belts. But it can also be used (with great success) for dress belts, briefcases, dress shoes, work boots, and numerous other leather goods.
This type of leather is often naturally marked with imperfections from the animal, like a brand or a scar, but products from pricier companies won't use these flawed hides — making it even more expensive.
Belt Leather Supplies and Full-grain is hard-as-nails and will develop a rich patina as it ages, looking more and more beautiful as you use it. It's widely recognized as the best and highest-quality leather money can buy.
Often much more expensive, full-grain pays dividends with its durability. If you invest in an item made with full-grain leather, you will probably have that item for the rest of your life if you take care of it properly.
One final word: If you can, you should avoid leathers like bonded leather (leather dust mixed with vinyl to create a strong leather-looking material), patent leather (leather treated with a glossy plastic finish), and corrected grain leather (flawed leather sanded down and finished with synthetic materials).