ajumma - a term used for a woman usually in her thirties/forties.
ajusshi - a term used for a man usually in his thirties/forties. The male version of "ajumma".
bias: Your most favorite member of a certain group or someone you are in favor of.
big three - SM Entertainment, YG Entertainment, and JYP Enterainment. These three companies have dominated the music industry for years, and while there are other companies out there that produce successful artists, it's the "big three" who are always expected to come out on top. Groups from the "big three" include Super Junior, Girls' Generation, Big Bang, 2NE1, Wonder Girls, and 2PM.
dongsaeng - a term used for one younger than onself. This can apply to either gender of both parties. "She's my dongsaeng" could mean "She's my little sister", or it could mean "She's my junior/younger classmate/younger friend, etc."
Hallyu - also known as the "Hallyu wave". It's basically a term for the influx of interest in Korean culture worldwide. For example, Japan is experiencing the Hallyu wave at the moment, as DBSK, BoA, Younha, and Big Bang have all found success promoting there. When someone says they're a "Hallyu star", they mean "international star", aka: known outside of Korea.
hoobae - a junior in the industry. ex: "Super Junior helped out their hoobae, Girls' Generation, and taught them the basics of the industry."
hwaiting/fighting - it means something to the effect of "Good luck!/You can do it!" Similar to Japan's "ganbare".
hyung - a term males use for males older than themselves. It loosely means "big brother".
maknae - the youngest in the group. Popular magnaes include KyuHyun (Super Junior), Seungri (Big Bang), ChanSung (2PM), SoHee (Wonder Girls), SeoHyun (Girls' Generation), TaeMin (SHINee), Minzy (2NE1), JiYoung (KARA), and GaIn (Brown Eyed Girls).
noona/nuna - a term males use for females older than themselves. "Big sister".
noraebang - the Korean word for karaoke.
omona - basically, it's the Korean version of "Oh my God!"
oppa - a term females use for males older than themselves. It also can be translated as "big brother".
selca - short for "self cam". This term is usually used when an idol/celebrity has uploaded pictures of themselves, taken by themselves. They're usually candid or very casual and quite different than photoshoot or studio pictures.
seonbae/sunbae - a senior in the industry ex: "Baek JiYoung is 2PM's seonbae. She's been in the music industry before they were even out of middle school."
It cane also mean senior as in mentor. ex: "It was awfully nice of Kim GunMo-seonbae to agree to take part in G-Dragon's solo album."
ssanti - cheap, as in cheesey or goofy. ex: "WooYoung's ssanti dance had everyone on Star King laughing."
trainee - a term used for a person in-training at a company who hasn't debuted yet. Usually it's hard to know how many trainees each company has, let alone who they are. Idol group members will often refer to their time as a trainee -- which can span from several months to several years -- and while it's usually perceived as a particularly grueling period of their lives in which training and practicing was all they did, they often say that it help define their character and personality.
trot - an old form of Korean pop music. In its most traditional form, it sounds similar to Japan's "enka". These days, half-pop/half-trot songs are more common, such as Park HyunBin's "Shabang Shabang", Big Bang DaeSung's "Look at Me, GwiSoon", and Super Junior-T's "Rokkugo".
unni/eonni - a term females use for females older than themselves. "Big sister".
ulzzang/uljjang - "best face". Someone who is known for being extremely handsome/beautiful. It's also a term used for those who, on the internet, become popular through posting pictures of themselves, usually editing their photos or applying effects such as eye tape, circle lenses, and other things of the sort that make them look more like dolls than humans.
Let me know if something need to be added.