What is World Youth Day?

A [recent question](https://christianity.stackexchange.com/q/14939/3941) mentions [World Youth Day](https://en.wikipedia.org/wiki/World_Youth_Day) . Wikipedia says that it's an event for young people organized by the Catholic Church. That isn't very helpful, since there are a lot of different types of Christian youth events. How is World Youth Day different from other events? Why do people go? What can people expect if they go? Who counts as a young person? What's the goal?