My T-MoneyI urge you to get the Seoul City Pass or T-Money card.
Seoul City Pass (as of now is 3,000 won each).
All types of T-money cards have a zero balance at the time of purchase. So, you need to top up the value before using the card. You can buy the card at any convenience store (with T-money signboard) or any subway stations.
It will save you a lot; time or money, if you come from Malaysia or other lower value currency countries.
1 - If you purchase one way ticket, it will charge you 1,000 won per ride, for the first 10 km. However, with Seoul City Pass you will only pay 900 won and goes up 100 Won every 5km after.
2 - You will receive some other discounts such as; 10% to N Seoul Tower, 10% to Lotte World, Free admission to Seoul Museum of Art, 10% to Nanta theater and many more. Once you purchase the Seoul City Pass, you will receive the discount booklet. So, use that!
3 - If you stay at touristy area eg Myeondong or Insadong, just walk to the nearer district like Myeongdong, Insadong, Jongno, Namdaemun or even Chonggyecheon. They are just near to each other. Read your map and go figure. Don't simply take the subway cause it will cost you 900 won each ride.
4 - Transfer discounts using T-money are available for transfers from bus to subway, from subway to bus, from bus to bus, from bus to shuttle bus (or maeul bus), or shuttle bus to bus. Discounts are not offered for transfers from subway to subway, however. Transfer discounts are offered only if your transfer is made within 30 minutes of your last transportation use. If you are transferring from a shuttle bus to the subway, you will be charged 500 won for the shuttle bus and 300 won basic fare for the subway. After that, you will be charged any additional fares for distances exceeding the the basic distance.
For example - I took a bus from Yeoido park to Ewha University station. After that take Subway from Ewha University Station to my hostel at Yaksu Station which cost me only 900 won.
For this, you need to learn HANGUL cause the bus schedule will look like this:
But I am glad that I took the time to learn.
5 - You can use the card to make local calls however you can only use on telephone which has T-Money sign on it. Hence, it is not necessary to purchase a separate telephone card. If you want to call oversea, use skype. It's super duper cheap. Only cost me MYR0.50 for 10 minutes talk with my family. I didn't rent any phone here!
6. It can also be used for taxi fares, buses, storage lockers, etc. Hence, save your time.
7. DO NOT FORGET - to touch the card against the panel/sensor before you drop off from the subway or bus! IF NOT - you will be penalized have to pay a full fare without discount for transfers or you may be charged the longest-distance fare as a penalty. If you are not sure, observe how the Koreans do it.
Check my post on how to calculate subway fare from Naver interactive map. If you use T-money, you will save 100 won immediately for every ride.