Thanks shotta35! I listed the UMTS and LTE networks in South Korea below. I am still a little confused on how to know if the device will be compatible to any of the carriers in Korea. I am a novice and I really don't understand what "bands", "UMTS", "GSM", etc. are. I see that some "bands" match with the D820 device, but the frequencies seem different (e.g. 2100 UMTS in South Korea). I appreciate your help!
UMTS networks in South Korea
KT Corporation/ South Korea/ 2100/ 1/ December 2003
SK Telecom/ South Korea/ 2100 / 1/ December 2003
LTE networks in South Korea
KT Corporation/ South Korea/ 850/ 26/ FDD/ TBD (planned)
KT Corporation/ South Korea/ 900/ 8/ FDD/ September 2013
KT Corporation/ South Korea/ 1800/ 3/ FDD/ January 2012
LG U+/ South Korea/ 850/ 5/ FDD/ July 2011
LG U+/ South Korea/ 2100/ 1/ FDD/ July 2013
LG U+/ South Korea/ 2600/ 7/ FDD/ December 2013 (planned)
SK Telecom/ South Korea/ 850/ 5/ FDD/ July 2011
SK Telecom/ South Korea/ 1800/ 3/ FDD/ June 2013
Nexus 5 (D820)
GSM: 850/900/1800/1900 MHz
CDMA: Band Class: 0/1/10
WCDMA: Bands: 1/2/4/5/6/8/19
LTE: Bands: 1/2/4/5/17/19/25/26/41