Leather is measured in ounces per square foot; the higher the oz, the thicker the leather.
"Grain" refers to the layers of the hide.
Full-Grain (pictured here) is the entire thickness of the hide and is the best choice for belts if durability is important to you. A good full-grain cowhide belt of 8 oz or more will last years beyond anything else. Wallets made of 4-5 oz full-grain leather will give similarly long wear, but will require a bit of a break-in period before they lose their stiffness.
Top-Grain is the top layer of the hide, shaved off with machines called splitters. It works well for wallets and small items, but will not last as long as the full-grain. (no picture, as I rarely use top-grain leather)
Split Leather (pictured below) is what is left after the top-grain has been removed. It has a suede surface on both sides and is often used to make moccasins and chaps.
Here comes the part where I don't mince words; Bonded Leather. Sounds good, doesn't it? Like it's certified or guaranteed. Such impressions are deceiving. Bonded, in this case, refers to the bonding properties of glue. Bonded leather is not "grain" leather, rather it is bits and pieces of leather ground up, then mixed with a bonding material and molded into a flattened product -- sort of the leather equivalent of particle-board. In my opinion Bonded Leather is a sub-standard product and should be avoided. It will not stand up to wear. I have no samples to show. Products made with this mock-hide are usually clearly marked.
There are other terms -- Vegetable tanned, chrome tanned, latigo and so on -- we'll cover those in another post. I'll close today with a picture of a hand-painted traditional Ojibwe pattern custom-carved belt made of 10 oz Full-Grain Vegetable-Tanned Cowhide. Happy Day!