* 어둡다 (eodupda): This is the most general word for "dark," referring to a lack of light. It's an adjective, so you'd use it like this: 방이 어둡다 (bang-i eodupda) - The room is dark.
* 깜깜하다 (kkamkkamhada): This word implies a very dark, pitch-black darkness. It's stronger than 어둡다 (eodupda).
The best choice depends on how dark you want to describe something. For most situations, 어둡다 (eodupda) is perfectly suitable.