This is an onomatopoeia for laughter. Its origin is the Korean onomatopoeia γ γ γ , in which γ stands for the "k" sound, like in raspy, stifled laughter. (I'm not sure why it wound up transliterated as "kekeke" rather than just "kkk", though.) Incidentally, the same onomatopoeia also exists in Japanese, written as γγγ.
Korean keyboards have a key that generates 'ke'. Someone discovered that this annoys the hell out of westerners, and due to South Korean Starcraft Zergrush skills, it's associated with Koreans online. Usually an expression of humor.
It's what some koreans say in online games, it usually signifies joy, or sometimes its like a warcry. Some people mock koreans with this word too. Can be used with words such as giev or maybe even itam/
