Korea, Seoul: T-Money Card

The best way to get around with public transportation is by using the T-Money card. It's a prepaid card that allows you to use the subway and buses within Seoul and a couple of other cities within South-Korea. You can also use it to pay in certain convenience stores.

The T-Money card comes in different kinds and shapes, the standard being a card the size of a normal credit card. You can also find shapes for your key chain or phone chain, always depending on where you buy it.

How to get a T-Money card? It's actually very easy as you can really buy it in almost any convenience store. On the internet it says that you are to look out for the T-Money sign but that is not always true. It's best to look for a convenience store in or near a station, go to the counter and ask the clerk for a T-Money card. They will then either give you one or send you to another store that does have a card.
The easiest way is probably to go to a store right at the airport as I am sure they have those cards there or at least know where to get them.

The card costs 5000Won and apparently you can get it refunded when you leave the country.

Once you bought the card, you need to charge it first. To do so, all you have to do is go to the station and look for a machine that says "Ticket Vending and Card Reload Devices".

First you choose your language and then click on the menu item at the right side.
Place you card at the recharging pad.
At the next menu, choose the amount you want to recharge. One trip in Seoul is about 1050Won - 1150Won for an adult.
Pay the amount and wait until recharging is done.

To use the card, just place it at the security card reader when entering and exiting. You will see two numbers at the screen. The number at the top indicates how much this trip will cost you, the number at the bottom shows you how much money you have left on your card.
If you try to enter although you don't have enough money, you will get an error and the security gate will close. You will then have to recharge your card first.
If you try to leave and you do not have enough money left on your card, you will also get an error and the security gate will close. But not worries, there are always recharge machines next to the gates inside the station where you can recharge as well.

More detailed information about the T-Money card and other cards and pictures are found at the official site of Korea tourism.