There are three class of bullet trains between Tokyo and Osaka.
- Nozomi
- Hikari
- Kodama
Your Japan Rail Pass covers Hikari and Kodama but Nozomi is excluded. Nozomi covers Tokyo to Shin-Osaka (552.6km or 345 miles) in about 2 hours 30 minutes. Hikari takes about 3 hours (depending on the number of stops) and Kodama takes about 4 hours.
Most trains require you to have seat reservation. You should be able to get the reservation at any station.
I recommend you seat at the north side of the train for two reasons. Sun shines from the south most of the time and you need to lower your shade to block the sun. North side, you get to see Mt. Fuji ... if you are lucky.