trains in Korea....

The KTX (short for Korea Train eXpress) is easily the fastest - but most expensive - way to travel by train. Offering a trip from Seoul to Busan in just under three hours at a top speed of 300 kph, you're paying for speed and convenience to major destinations. KTX trains make fewer stops, but run more trains to the major destinations than the other classes of trains. A new high-speed line is currently under construction in southern South Korea; when completed at the end of 2010, it will reduce the Seoul-Busan trip time to approximately 2 hours and 10 minutes. (Source: Wikipedia)

The Saemaeul-class (새마을) trains can be considered Korea's second-class trains. Until recently, this was the fastest way to get from point A to point B, and still maintains a good balance between speed and number of stops. The name 'Saemaeul' comes from the Saemaeul Undong, a movement in the 1970's to make rural South Korea more modern.

The Muganghwa-class (무궁화) trains can be considered Korea's third-class trains. Although these trains make more stops than the other two classes of trains combined, the train's comfort and cost of tickets may actually make this the preferred mode of transportation for some. While these trains will take the longest, these are the only trains that stop at some train stations. The name 'Muganghwa' comes the national flower of South Korea of the same name.

The Tonggeun (통근) or 'commuter-class' trains only run a couple specific routes. These trains will make many stops during their runs, and only serve smaller or more rural communities that have a train connection. The few lines that still use Tonggeun-class trains primarily run to the northern Gyeonggi-do areas, including the Gyeongwon Line (Dongducheon-Sintan-ri) and the Gyeongui Line (Munsan-Dorasan). Don't confuse this with the subway line of the same name - that line terminates at Munsan. To go any farther north you'll need to hop on the Tonggeun train.
The Nooriro-class (누리로) is a new class of trains that compromises between the Muganghwa-class train and the comparatively slower Seoul subway. While it runs on the same line as line 1 of the subway system, it makes fewer stops than the subway does (stops include Yeungdeungpo, Anyang, Suwon, Pyeongtaek, Cheonan, Asan, Onyang Oncheon, and Sinchang). While you pay the same for the Muganghwa-class train, you get there a little faster. This class of service is expected to replace the now-aging Mugunghwa trains, but they don't yet seem to have a lot of demand.