The cathedral stands on a wide open square called Red Square, right in the heart of Moscow. Its official name is the Cathedral of the Intercession of the Most Holy Theotokos on the Moat, but almost everyone calls it Saint Basil's - named after a holy man called Basil who lived nearby. The building is made up of nine separate chapels, all joined together, each with its own differently shaped dome on top.
The brightly painted domes are the most famous thing about the cathedral. They are called onion domes because of their rounded, pointed shape - a little bit like a big onion sitting on top of a tower. The domes were not always so colourful; they were originally golden, and the bright painted patterns came later, making the cathedral look even more like something from a fairy tale.
Today Saint Basil's is a museum, so visitors can walk inside and explore all nine chapels. The inside walls are covered in beautiful painted patterns and religious pictures called icons. Red Square around it is often filled with people, and in winter a big outdoor ice rink is set up nearby - so you can skate with one of the world's most spectacular buildings right in front of you.