A Motte-and-bailey is a kind of castle, or fortification. Many of them were built in the 11th and 12th century around Europe. These structures were made on a hill (that was often artificial). On this hill, a keep of wood or stone was built. This was easy to build, and the materials were readily available and cheap. However, this meant they were easy to burn down. Also wood rots when it gets wet so the structure doesnt often last long and need lots of repairing. They used to be made out of wood but as time went by different types of castles made of stone became more common as they were much stronger.These were surrounded by a ditch and protected with a stone wall.
Sometimes the hill was already there, like in Castelnou
Motte in Saint-Sylvain-d'Anjou
Sometimes castles were built on the site of the motte, like in Gisors