These structures were built on a hill (that was often artificial). On this hill, a strong keep or "motte" of wood or stone was built. There was a larger enclosed courtyard, or "bailey" surrounded by a protective ditch and palisade. They were quite easy to build with unskilled, often forced labour. All the same, they were significant and difficult to overcome.
If they were built of wood, this meant they could be burnt down. Also wood rots when it gets wet so the structure doesn't often last long and need lots of repairing. As time went by castles were made of stone, which is much stronger. These were surrounded by a ditch and protected with a stone wall.
Motte and bailey castles appeared in England after the Norman Conquest of 1066. They were a common feature in England by the death of William the Conqueror in 1087. Their construction was the start of what was to become a massive castle building programme in England and Wales. Motte and bailey castles have been around for eight centuries and are part of today's history.