Interrail is a service in Europe. It allows you to travel throughout Europe by train. There are different types of tickets that are explained here.
This service is only available to European citizens or people having their place of residence within Europe. If neither applies to you but you still want to travel Europe by train, you can do so with Eurail.
Depending on the ticket you purchase, you can either travel all of Europe, or a single country of your choosing. It is fairly easy – once you have your ticket, you can decide where exactly you want to go. The interrail website is a big help with the planning. On it you can find the different types of trains you can take, and whether or not you need to reserve train seats.
Interrail is perfect for people of all ages – as long as you are older than 18.