Jirisan National Park
Jirisan National Park is the largest - and perhaps most famous - national park in South Korea. It spans three provinces: South Jeolla, North Jeolla and South Gyeongsang. Within the park is Jiri mountain, which is considered one of the three holy mountains on the Korean peninsula. It offers some of Korea's best hiking and is the beginning of the Baekdu Deagan (백두대간) Ridge, the "spine" of Korea, which extends all the way to Baekdusan (백두산) on the border of China and North Korea. The park has 12 peaks over 1000km with a 40 km ridge. The highest peak is Cheonwangbong (1915), which is South Korea's second largest mountain.
Jirisan became Korea's first national park in 1967.
Flora and fauna
From Busan: Jirisan is best accessed by car or bus. By bus, depart from Seobu Terminal. Two buses run daily between Busan and Sanggyasa, but the last bus departs Busan at 16:00 - not ideal for the working folk. You can also take a bus to a nearby city and transfer to a local bus. Consider any buses going to Hadong, Gurye, Namwon, Hamyang, Sancheong, or Jinju. If possible, call ahead to determine the bus schedule from those cities. Buses usually run pretty regularly, but some routes stop earlier than others.
Sanggyesa Temple: This is one the major temples in Korea. You can take a bus from the Hadong Terminal.
Buril Waterfall: This waterfall is only 1 or 2 km away from Sanggyasa temple, but it is a decent climb.
HIKING! That's what everyone is here for. The park office recommends a several courses. Consult the official website for details. 
Fresh water springs are relatively consistent along the trails. The mountain water is filtered through granite and is safe to drink, according to every Korean voraciously drinking it. Beware that some trail maps mark water springs that do not (or no longer) exist. Have an extra full bottle just in case.
The area, specifically Hadong, is known for green tea. Shops and stores around Jirisan usually sell tea leaves by the bag. If you are a true tea fanatic, come in May for the Hadong Wild Tea Cultural Festival.
The small villages near the park entrances offer various lodging options. You can bet on finding a minbak (room rental) for a somewhat reasonable price.
There are several campgrounds at various locations. Fees are minimal (1,600-2,000won/adult) and reservations are not necessary. Consult the Korean National Park Service website (in Korean and English) for specific campground locations.
You also have the option of staying at a mountain shelter. At the time of writing, the park has 6 mountain shelters. Reservations MUST be made in advance via the park website. If you call the office, they will just refer you to the website. Reservations can be made in English here:  During non-busy periods, two days notice is sufficient. However the spots fill up fast during holiday weekends, so try to plan ahead.
Fees for the shelter are 5,000-8,000won/adult, depending on time of the year and specific shelter.
Camping is forbidden within the boundaries of the national park, except on designated camping grounds. Throughout the park, signs are posted giving warnings of a 500,000won penalty for violations. Hike to your heart's content, but laying down to sleep is a risk. For those of you considering sneaking off trail and pitching a tent illegally, be careful for snakes and bears.