For tourists going to Japan, the Japan Rail (JR) Pass is really useful if you are going for 1, 2 or 3 weeks to Japan. It allows you to take all the JR lines (except the Nozumi and Mizuho trains) at no extra cost, other than what you pay for the pass. It is only valid for non-Japanese, and must be purchased from outside Japan.
I bought my JR Pass from Singapore, from Nippon Travel Agency in Singapore. I placed a booking at their website and they delivered the Exchange voucher within 3 days, using the Ta-q-bin delivery service (at no extra charge) that collects the money on delivery. I managed to get the Pass voucher at $435, which was the cheapest for Feb to April, and it was just nice that it was a few days before my trip to Japan.
And to my astonishment, it came with so many gifts! There was an Osaka Guide, 5 Tokyo area maps, and also a piece of cloth, that I think is used to wrap bento boxes. All fitted into an envelope, and the girl who did the delivery (normally is a guy) said "such a thin envelope and it's so expensive" when I passed over the $435.
When I reached Japan, I went to the nearest JR Station and switched the voucher for the Pass. Each pass has the date of expiry stamped on it and you can make reservations for the Shinkansen (bullet train) free-of-charge. You just showed it at the manned gate at each ticket entry area, together with the reserved Shinkansen ticket.
This pass can be used for entry into local trains in Osaka, Kyoto and Tokyo (and likely other big cities as well). It is especially useful when you plan the location of your hotels/destinations to be on the JR Lines and can save money during the trip. Japan's train/subway travel expenses are really expensive, if you compare it to that of Singapore, but their trains are relatively on time, and rather frequent. Most of the time if there are disruptions in the train lines, it is due to weather problems (like earthquakes, typhoons, snow, strong winds). As their train tracks have mostly 2 tracks, they are able to provide express trains for some longer journey trains.