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 ¥2,500 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's quite a hassle to do so, 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.