The T-Money Transit Card can be used for buses, subway, taxis, and even to pay at chain convenience stores. It is a prepaid, rechargeable card.
T-Money cards can be purchased at ticket machines in subway stations or from convenience stores (CU, GS25, 7-Eleven, etc.). You can only use cash to buy the card at the subway station, but you can pay by card at convenience stores.
Card price: Starting at 4000 won (depends on design)
You can recharge your card (with cash only) at convenience stores or at ticket/recharge machines (near the gates) in subway stations. You can also recharge them directly at an ATM from your bank account.
Basic fee - 1250 won
Basic fee for blue/green buses: 1200 won (card), 1300 won (cash - give as exact an amount as possible, the bus driver can only give change in coins).
You can transfer for free to another bus (cannot be the same number bus as the one you got off from) or subway. You must tag the T-money card on the reader when getting off, otherwise you will be charged a full fee at the next stop.
You can transfer between buses and subway up to 4 times. The next leg of the trip must begin within 30 minutes after getting off the previous means of transportation. After 9 pm until 7 am the transfer time is one hour. You cannot transfer from subway to subway for free if you go through the gates and tag out.