A guy is an informal word for a man or boy. You might say “that guy over there” when pointing out someone at the playground, or tell your friend “he's a nice guy” when describing your neighbor. The word feels casual and friendly, the kind of thing you'd say in everyday conversation rather than in formal writing.

Interestingly, guys (plural) has become so common that many people use it to refer to groups of people regardless of gender. A teacher might say “okay guys, time for recess” to a whole class, or you might greet your friends with “hey guys!” even if the group includes both boys and girls. Some people don't like this usage because guy originally meant only males, but in casual American speech, you'll hear it used this way all the time.

The word can also appear in phrases that describe types of people: a tough guy acts strong and fearless, a bad guy is a villain in a story or movie, and a regular guy is someone ordinary and down-to-earth. When someone says “guys like him,” they mean people similar to that person.