Like the Japanese word maru/kuruwa implies, baileys are literally a circle around the main keep. Castles can have any number of baileys, but generally they have at least three basic ones. These are called the honmaru (main bailey), ninomaru (second bailey) and sannomaru (third bailey).
The diagram to the right shows a very general idea of what the layout of a castle looks like. The star in the center represents the main keep. Each castle is unique in its own design so each bailey is not perfectly positioned in the center of the next bailey. They are usually off set in one direction or another and positioned to work with the landscape in a manner that strengthens the castle further. Sometimes they may all share one side but each bailey goes out farther than the previous one making rings around the castle. The picture above shows the honmaru as viewed from the top of Kochi-jo. You can see that walls make a ring around the main keep and work with the yagura seen here to protect the castle.
Castles are not limited to these three concentric baileys, but often have other baileys inside these three or baileys connected to the outside too.