Suica and Pasmo are very convenient pre-paid smart card. Just one card allows you to use railways and bus routes nationwide as well as shopping at convenience stores and vending machines. All you need to do to go into the station is to pass the card over the card readers at the ticket gates. It is especially handy when you travel in Tokyo and its vicinities where you need to make a lot of transfers between different railways. You do not have to go through the trouble of buying tickets every time. Suica and Pasmo are interoperable, so just get one of them and you are good to go ! It is highly recommended to buy one at the train station adjacent to Narita or Haneda Airports as soon as you arrive at Japan.
A lot of foreign visitors traveling in Tokyo often stand still in front of the auto ticketing machine, with absolutely no idea what to do. That is because not only they do not read Japanese but also each railway operator uses a different route map. And when they get on the “subway”, they cannot tell if they are on Tokyo Metro Subway or Toei Subway. While we often see self-admiring comments like “the railway network in Japan is the best in the world,” the network in Tokyo is too complicated for foreign visitors to make the full use easily.