The PASMO IC card is a pre-pay card you can use in the Tokyo area for most buses, trains, and subways as well as in some stores. The "IC" stands for "Integrated Circuit" or "Integrated Chip" as the card contains an embedded chip that remembers your balance.

This page describes two pre-pay cards: PASMO and Suica. In the Tokyo area these cards are interchangeable. For this trip you will get a PASMO card with ¥4,000 already on it.

There is a home page for PASMO here. Wikipedia has a pretty comprehensive article on PASMO. Another helpful site.

Note that when you are done, you can claim any unused balance on your card. Frankly, it looks to be a bit of a hassle, and we don't expect the remaining balance to be very much. My thinking is just keep it as a souvenir or give it to someone local for their use.