'ㄱ' is a velar stop and can be voiced or voiceless depending on the phonetic context. Put simply, it's a 'k' that turns into a 'g' when it occurs between two vowels. 'ㅋ' is also a velar stop but is more tense, heavily aspirated (i.e., pronounced with a strong breath like making an 'h' sound), and never voiced (i.e., it can't end up sounding like a 'g').

they has different pronounciation. "ㄱ", called "Gi Yeok" is used to pronounciate "G" and "ㅋ", called "Ki Yeok" is used to pronounciate "K" for an example, "Go" is wrote "고" with "ㄱ" in korean. one thing you have to keep in mind is that "Gi" doesn't read "zi" in korean. In korean, you have to read it with ㄱ. and "Ko" is wrote "코" with "ㅋ" in korean. and also korean netizens use "ㅋ" to express their laughing. "ㅋㅋㅋ" means LOL in english. in this use, "ㅋ" comes from sound of laughter like "KiKiKiKi"