ONE-DAY PASS
To note, there are different railway companies that offers different types of passes, so you should probably plan where you want to go before you decide which ticket to purchase. If you are moving around a lot in one day and not sure what your plan is, get TOKYO 1-DAY TICKET which pretty much covers major lines.
These 4 are the most convenient passes.
JR PASS
www.japanrailpass.net
JR PASS is like a golden ticket. JR bullet train, shinkansen is probably the easiest way to get outside of Tokyo, but IT'S FREAKING EXPENSIVE. For example, Round-trip from Tokyo to Osaka is ¥27,240 and the cheapest pass (valid for 7days) is ¥29,111. See how valuable it is? This great deal is offered only to foreign passport holders, and you should definitely take this advantage to explore.