Hi!
If you are planning to travel to South Korea, you have to get a transport card, as it will help you to use the bus, the subway, taxis and some trains including limousine from Incheon airport to Seoul and back.
In South Korea there are several transport cards which also serve as T-Money. Arriving at the airport you can buy it in the convenience stores. Also, if you buy a transport card you can forget to buy a ticket for each time you get on public transport, you just have to have enough balance. Once you have your card, you must recharge it at the machines that are at the airport or at any subway station
There is a special travel card for tourists, which also offers you discounts and free tickets to many attractions in Seoul, museums, palaces like Gyeongbukgung, tour bus, Namsan Tower, etc. Here is the link for more information: Discover Seoul Pass
If you are going to use the subway, you only scan it before passing the turnstiles
If you are going to use the bus, you must scan your card in the machine that is next to the driver when getting on the bus and you should never forget to scan it before getting off the bus, it is very important that you pay the exact amount of your trip.
Have a nice trip in South Korea!!
Gyeongbokgun Palace