Capital assets are typically non-“ordinary course of business” in nature. This includes stocks, bonds, your personal residence, your car, jewelry, etc.
Non-capital assets are business in nature. Inventory, AR, equipment, or items held for inventory (this is key. property held for resale is non-capital since it's held as if it's inventory).
A 1231 asset is a non-capital depreciable asset (real or personal) that has been held for a year or more. For farms, this includes livestock. These have special tax treatment in that if sold at a gain, you can treat the gain as a LONG TERM CAPITAL GAIN. If the item is sold at a loss, you can treat it as an ORDINARY LOSS and offset ordinary income. This is a big deal because you get the benefit on both sides of the sale (gain or loss).
Inventory is a non-capital asset, but it can NEVER be a 1231 asset.