1) Seoul Subway - Interactive maps:
a) Seoul Metro - http://www.seoulmetro.co.kr/station/eng/linemap.action
b) Seoul Metropolitan - http://www.smrt.co.kr/english_smrt/index.jsp
c) Standard KTX tickets (http://www.korail.com/en/rv/pr21100/w_pr21110.jsp) - This is the second quickest option to get to places like Busan from Seoul, after flying. A standard KTX (non-business class ticket) is also roughly 30,000 won cheaper than a flight ticket (+-55,000 won), but it takes 2 to 3 hours longer to reach your destination. On the plus side, the KTX drops you off right in the middle of Busan, with direct access to the Busan subway, whereas if you fly to Busan, you will have to take a Shuttle bus, or taxi from the airport to get to the center of Busan (which can actually take quite a long time if you travel over busy peak times, so this is worth keeping in mind if you want to get to somewhere in the middle of Busan quickly). As with flight tickets, you can just arrive at the station to book an available seat, but be advised that KTX tickets sell quickly over busy periods such as weekends (especially if the weather is great). The KTX also offers additional services such as a movie carriage. Check before you book this carriage (for about 65,000 won), since not all the movies are in English.
Korea Rail Pass (for foreigners) (http://www.korail.com/kr_pass.jsp)- This is a very good option if you are planning to travel to another city for a short gettaway, or want to travel extensively over a period of 10 days. You require a creditcard, a passport, and can book a ticket online (or through certain travel agents). For about 85,000won, you can make use of the KTX and other rail services, as much as you want over a period of 3 days for example. Considering that a single train ticket from Seoul to Busan can be 55,000 and up, and a flight about 80,000, this is not a bad deal at all. The great thing about the ticket is that you can book it in advance, and if you do not use it (i.e., exchange your eticket for a real one), then you wont get charged. So it is an excellent backup to have, for in case you might want to travel.
d) Bus Shuttle (http://english.visitkoreayear.com/english/benefit/benefit_07_01_01.asp) - If you plan to travel between Seoul and Busan, the cheapest way to get there is via this service, since it is free. :)) The only downside is that you apply for a ticket, and your name then goes into a draw, which means, that only if you feel lucky punk, you might get a ticket. Also it is a bus, it will take you quite a few hours to get to Busan (5 hours and longer if i remember correctly)...but still, it is free.
e) Domestic flights with Korean Air (http://www.koreanair.com/) - Flying from Gimpo International Airport to Busan, is pretty much the same as flying from Cape town to Johannesburg. It takes roughly a hour to fly between the two cities, but add more time for checking in, etc. Over non-peak periods you can just arrive at the airport, and buy a ticket for around 80,000 won... however, over weekends, you might have to wait a few hours before you can get an available seat. You can book/reserve a ticket online in advance (which is advisable), but you will require a credit card to do so.
f) Interactive bus map: