If you buy a Japan RailPass, you get unlimited travel on all JR trains anywhere in Japan for 7, 14 or 21 days (at increasing cost). The RailPass can also be used on a special ferry and a few special bus routes. You can also buy cheaper ‘regional’ RailPasses for only a part of Japan. Unfortunately, the main tourist route from Tokyo to Osaka spans two such regional passes, so it’s cheaper to buy a RailPass for the whole of Japan, rather than two regional passes.
JR trains are the above-ground trains in most cities of Japan, and also almost all intercity trains are JR. They include the famous intercity ‘bullet-trains’. So a RailPass gets you unlimited bullet-train travel for a period of time. (But note that not every bullet-train is covered by the RailPass – generally, the fastest bullet-trains cannot be used).
IMPORTANT – JR does NOT run subway trains, so you can’t use your Railpass on the extensive subways in cities like Tokyo or Osaka. For those, it’s best to also buy an IC card.
A RailPass is not cheap – the cheapest RailPass (7 days, Economy Class) is about $500. You can read more about RailPasses and see the current prices here. But country rail-travel (including bullet-train) is fairly expensive anyway. So you should add up the estimated cost of all your intended rail travel and calculate if it would be cheaper for you to buy a RailPass.
You can activate your RailPass to start on any day – it doesn’t have to start the day you arrive in Japan. So you can structure your rail travel into the time period you have an active RailPass.
Japan Tourist Tips 