There are two different classes of JR Pass: Ordinary and Green. The Ordinary Pass is valid for travel in Ordinary cars both reserved an unreserved.
The 'Green Pass' is valid for travel in the ‘Green Car’, which is the equivalent of First Class, on Shinkansens and certain lines that provide a First Class Carriage.
The benefits of the Green Class Japan Rail Pass are that they offer more comfort than Ordinary class seats, more space and usually quieter travel. The Green Class JR Pass is good value for the extra level of luxury and comfort offered. If you are planning to travel during peak/busy times, the Green Class Japan Rail Pass could be a good investment. This is due to each Green Car seat being larger, giving each passenger more space and Green Cars are generally less busy.