Something famous is well known, usually because it's special or particularly good. Celebrities, historic paintings, and the best burger in town could all be described as famous.
If something is famous, then a lot of people know about it. If you turn on the television or look at a magazine rack, you'll see plenty of famous people: movie stars, TV stars, musicians, athletes, and politicians are all famous. Lots of people want to be famous, but there are good and bad parts to it. It must be nice to be rich, but being constantly surrounded by fans and photographers can't be fun.