http://www.jreast.co.jp/e/pass/suica.html ( Suica - watermelon -green)
http://www.pasmo.co.jp/en/ ( Pasmo - pink )
http://www.tokyometro.jp/en/ticket/types/index.html Additional Info on the cards & tickets
If you are visiting Japan a few years ago there might have been some differences between these two cards but as of now you can use these two everywhere you go ( trains/buses/vending machines/stores), it's almost like a credit card. It used to be you can only purchase items at certain stores ( pasmo at pasmo store and suica at suica store) but the two have merged and that makes it very easy for us to navigate around. I would recommend purchasing the card immediately upon exiting from the airport if you are taking the train from the airport to your designated location in Tokyo, because you might have to make several transfers before reaching your destination.
The cost for the card is 1000 yen(~$10), but the fee for the card is 210 yen and when you returned the card you will get back 500 yen. I used Pasmo the whole time I was there but my friends use Suica and we've never run into any problem with them. Keep in mind that this is only for Tokyo and Kanto region, you will need a different card if you're studying or traveling to Kansai ( Kyoto, Osaka ) region. I traveled there for one week so if you have any question about that, just send me a message :D
P.S I wanted to be cool like everyone else and put my card in my wallet and walk through the machine or using it like my friend is below but sometimes the thickness of your wallet or the fact that you have too many cards in there could prevent the card from being properly scan so keep that in mind.