insan means human, kişi means person but sometimes insan is also used as person. example (similarity): "orada çok insan var mı?" = "orada çok kişi var mı?" = "are there many people?" example (difference): "insan bencil bir varlıktır." = "human is a selfish creature." (we cannot use kişi here; because kişi is not a creature) "bir kişi sizi görmeye geldi." = "a person has come to see you." (we cannot use insan here; because it would be weird to say "a human has come to see you" -as if an alien could have come too :D ok it can happen in a movie, why not -)