Seishun Juhachi Pass allows five days unlimited use of local train (not express) for the nationwide network of Japan Railways. This is sold both to Japanese and foreign visitors. Five days validity can be separate or consecutive. You can also share the Pass with others as long as you pass ticket gates together. For example, a group of five people use one pass for a day.
The price is 11,500 yen. Comparing with the regular fare, traveling from Kyoto to Tokyo costs 7,980 yen, (about nine hours and six times connections!) but with the Pass, it is only 2,300 yen (11,500 yen divided by five days). The pass is only available within 40 days in spring, 50 days in summer, and 40 days in winter.
JR’s web site, http://www.jreast.co.jp/e/pass/seishun18.html has details.
If you have the JR Pass, Seishun 18 Pass is of course no use.
The map below shows the regular fare of JR local train. If you ride as far as 2300 yen from Kyoto station, Seishun 18 Pass would be worth it.