Leather comes is many types and forms. It can be in its natural external layer form known as full-grain leather where the leather is most durable and enduring. It can be top-grain leather that has been sanded down into a less durable type known as "top-grain leather", which isn’t as durable as full-grain leather but also lasts a long time. It can be the second layer of leather, which is the under side of the full-grain/top grain leather layer, which has been known to deteriorate much faster than the first layer. It can be sheep leather, and it could even be PU (a synthetic material known to be used in place of leather and often called and labeled as “leather”).
In all cases, buying a leather bag of really good quality depends highly upon your ability to understand the differences between leather types.