man = 1 male person men = (plural) 2 male people or more

The other answer are right (man is singular and men is plural). But also "man" is sometimes used to mean "mankind" (that is, humans). Therefore, the following sentences are BOTH correct and mean the same thing: Man has been to the moon. Men have been to the moon.

men is plural man is singular

Note: It's polite to say thank you to people who have answered your question.