If you’re staying in a bigger Japanese city, public transportation is the way to go if you want to get from A to B as quick as possible. Since having a car is expensive and annoying when it comes to parking and traffic, you’re better off using one of the many transportation services which are offered to you, such as the subway, trains or busses. 
Japanese public transportation services can be payed with cash but it might be a struggle to always buy a ticket at a counter so there is a much easier and handy way to pay for your train rides: the Suica (スイカ)

Suica is a rechargeable contactless card for electronic money used to pay fares. Launched in November 18, 2001, the easy usability of the card made it gain success all over the country fairly quickly. Kyūshū and the entire Kansai area started out introducing the card and integrated it into the transportation system and many prefectures and areas followed their example. In Tokyo, the Suica is called Pasmo card but both systems are basically the same and you can even use a Pasmo card in Suica card areas and the other way around. The card is also increasingly being accepted as a form of electronic money for purchases at stores and kiosks, especially within train stations. As of October 2009, 30.01 million Suica cards are in circulation.
Suica stands for “Super Urban Intelligent CArd”, and the pronunciation is also a pun on the Japanese word for watermelon, “suika”. Using a Suica is fairly easy: you charge them with money at a station ticket counter and every time you enter a train station you just hold your card over the station’s gate which will open the doors. After leaving the train at your destination, just open the exit gates the same way and your ride will automatically be paid. The chip inside the card is so strong that you can keep the card in your wallet and just put your wallet on the gate scanner in order to process. 

The balance on the card is displayed when you enter the ticket gate this way. The minimum fare is needed on the card when entering the train system, which is not deducted at that time. The balance is also displayed whenever the card is inserted into the ticket or fare adjustment machines as well. A travel record is stored on the card, and can be displayed or printed out at the same place where one can purchase and reload the Suica cards.
If you stay in Japan, a Suica will definitely come in handy if you don’t want to keep a ton of change in your pockets.