A "guy" is a man. It's a casual word.
You use this word instead of "man" when you're talking to your friends or in a casual situation like at a party. The person you're talking about can either be someone that you know, or someone who you don't know but don't have high respect for.
For a stranger who seems older and more respectable, "man" or even "gentleman" are more polite.
Note that you can use the plural form, "guys", when you're talking to a group of men or women:
But "guy" (singular) always refers to a man.