Just like English, the Korean language is full abbreviations and slang words for making texting on your phone and chatting on the Internet faster. Those abbreviated words can be a real time saver and a lot of people, myself included, use some of these on a daily basis. And even though you may not want to use them yourself, knowing their meaning is always helpful. So here are the most common ones:
ㅋㅋ – sound of giggling/laughing.
ㅠㅠ / ㅜㅜ – sad/crying eyes.
ㅎㅎ – [하하/haha] – Haha.
ㅇㅇ – [응/ung] – Yes.
ㅉㅉ – tsk tsk (that clicking sound you do with your tongue when you are displeased with someone).