Japan Rail Pass is one of the necessary items to travel in Japan. It is valid for almost all trains operated by Japan Railways Group; JR Hokkaido, JR East, JR Central, JR West, JR Shikoku and JR Kyushu.
JR group is operating in all 47 prefectures except in Okinawa prefecture which has only one monorail line operated by non-JR company. Main purpose to use the Japan Rail Pass is to ride Shinkansen, the bullet train.
However, it is not valid for Nozomi trains, the fastest and most frequent trains of Tokaido Shinkansen, which is connecting Tokyo and Osaka. Along Tokaido Shinkansen is Odawara; gateway to Hakone, Nagoya; the fourth largest city in Japan and transfer station to Takayama and Kyoto; Unesco World heritage.
Therefore passengers of Japan Rail Pass must use slower and less frequent trains; Hikari or Kodama. However, the time difference between Nozomi and Hikari is just 30 minutes between Tokyo and Shin-Osaka stations.
Japan Rail Pass should be bought before arriving in Japan at the authorized agencies. The authorized agents will issue the exchange orders which you can exchange to valid Japan Rail Passes at the designated ticket offices in Japan with valid dates.
Please note that there are companies or websites which sell Japan Rail Passes even if they are not authorized agencies. Theses companies buy the Japan Rail Passes from authorized agencies and sell to the clients. Therefore it will take time to deliver the passes or may be sometime is much expensive than purchasing from the authorized agents.