The first three refer to an individual. The first two are a little more formal and are used either in academic contexts are by aliens. "All humans/human beings are born with two lungs." "A man" refers to an individual male. "Talk to that man to get directions to the hotel." "Man" and "mankind" refer to all humans but "mankind" is used more in this way than "man". "Man/Mankind has come a long way since the Dark Ages."

Yes, it can be confusing. Here are some sentences for context: Human - I made a mistake, I am only human. (describing 'self' essence as character - male or female) Human being - Don't talk to me like that, I am a human being! (state of humanness, being a member of the human race - male or female) Person - You are the type of person I would like to be friends with. (single individual, either male or female) Man - You are the type of man I could see myself in a relationship with. (adult male) Man - It is man who has polluted the planet (as in people as a whole - male and female) Mankind - One small step for man, one giant leap for mankind. (every person, as a collective) Does this help?