ㄱㅋ ㄷㅌ ㅈㅊ ㅂㅍ In each pair, the latter is more aspirated. For example, ㅋ will have a puffy whoosh of air, while ㄱ will have little to no puff. Same with all the puffs. You could say this 김 is said more like cim (no puff, or kim) and 킴 as kim (puff, or k'im)

http://koreanwikiproject.com/wiki/index.php?title=Learn_hangeul Take a look at that site. There are audio files where you can listen to the difference between all characters. In short, you let out a little bit more air when you say ᄏ.