A monorail is similar to a train in that it travels along a fixed track. The main difference is a monorail has only one track, not two like regular trains. Some kinds of monorail hover above their track (also called a rail or beam). These are called maglev monorails.
Many monorails travel on raised tracks above a city. Not all monorails are raised like this. The beams a monorail travels on are large and solid and are often cast out of concrete, like the raised platform itself.