For domestic travel (travel within Korea), rail and bus are the most convenient and affordable options. We’ve catalogued resources on these topics under transportation.
The one exception to this rule is travel to and from Jeju Island, a popular vacation island south of the Korean peninsula. Frequent and affordable flights connect Jeju to Seoul (Gimpo Airport) and also to most provincial capitals.
For international flights, the primary hub is Incheon International Airport, which services Seoul and is located about an hour east of the capital.
Smaller provincial airports may offer limited flights to nearby international locations, especially in Japan.
For accessing Fukuoka Japan, the most popular visa run destination, high-speed ferry may be cheaper and more convenient than air travel.
Incheon International airport is the largest airport in South Korea and the primary airport serving the Seoul area. Although the world’s eight busiest airport, Incheon is remarkably efficient, with average wait times of 16 minutes for departure and 12 minutes for arrival process. (To play it safe, though, you should still arrive for an international flight at least 90 minutes early). For seven years in a row (2005–2012), Incheon was rated the best airport worldwide by Airports Council International.
Located 70 km (43 mi) west of Seoul, Incheon offers easy access to the capital, as well as most locations in Seoul. The Incheon Airport line, which runs to Seoul Station, takes under an hour and costs about 4,000W. An express train, which runs every half hour, completes the same journey in about 40 minutes and costs roughly 14,000W. If you have a lot of luggage, you might want to try one of the dozens of limosine bus lines which run to locations throughout the country and cost around 14,000W. For more information, visit the link above.