A battlecruiser was like a battleship. It had thinner armor and lighter guns and was longer and less wide. They were faster than battleships, which meant they were made to go faster than bigger ships and chase down smaller ships.
Battlecruisers were used in the 20th century. The Royal Navy invented the battlecruiser, and soon later they were used by many countries around the world. They have been replaced with more modern ships which means they are no longer used. Battleships and battlecruisers are not used today because guided cruise missiles are much better than the guns battleships and battlecruisers had.