On the other side of the coin, you refer to someone as -kun when you're more senior, or older, than them. It's almost exclusively used towards young boys and teenage boys, but not always - for children/teenagers, it's the male equivalent of -chan.
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-san is most closely related to "Mr" or "Mrs" or "Miss", but you use it for anyone regardless of gender or marriage. It just denotes respect for the person you're speaking to - gives it a formal air. Generally you'd refer to someone you don't know as <last name>-san.