If you are used to taking the subway/MRT in any city, the concept is similar. Although the Korean subway system is more intricate, the lines are color coded, and stations numbered; so once understood, it is fairly easy to use for traveling. English names are also provided together with their Korean names.
Ticket machines work in the same way as those in other cities, and come in English instructions as well. Tickets are inexpensive, and the trains are clean. But try to avoid the rush hour between 5pm-8pm as trains can get very crowded.