A butt joint is a weak joint made by joining two pieces of wood perpendicularly using glue only. Just as the name implies, a butt joint is made by butting a couple of wood pieces together. The end grain of one of the two pieces of wood is set against the long grain of the other. Because nails and screws would likely cause the woods to crack or split, butt joints are given no reinforcements, which means they are very weak and not frequently used.
Just in case some fasteners are going to be used, most carpenters tend to pre-drill the holes where the fasteners fit in. It is very vital to cut precise squares when making butt joints. This would make it possible for the wood pieces to fit flush with each other. Because of the very weak nature of these joints, they are only used in pieces of furniture that will hold lightweight items and not be regularly moved.