The biggest difference is that くん is the suffix used for boys in common. さん is used for both men and women and more polite than くん. さん is the suffix most commonly used in both schools and business. For your information, ちゃん is the suffix used for girls in common.

Yeah, like Chiharu said, "san" is basically the equivalent of Mr./Ms./Mrs. "kun" is used for little boys and "chan" is used for little girls. This being said, I've been called "Jorge-kun" by some older women even though I'm 25. I asked why and they said because I was cute... (grr)

1.the difference of familiarity 一郎！　is the best relationship 一郎くん is so-so relationship 一郎さん　is stranger 2. the difference of gender 一郎くん male 一郎さん　female (hmmm....） 花子さん　female :) Sry my poor English