synonyms - What is the difference between "human," "human being", and "humankind"?

Those words all mean ..... WE!

Answer

• human: a human being, especially a person as distinguished from an animal or (in science fiction) an alien


• human being: a man, woman, or child of the species Homo sapiens, distinguished from other animals by superior mental development, power of articulate speech, and upright stance


• humankind: human beings considered collectively (used as a neutral alternative to “mankind”)

The NOAD has also these notes.

Traditionally, the word man has been used to refer not only to adult males but also to human beings in general, regardless of sex. There is a historical explanation for this: in Old English, the principal sense of man was "a human being," and the words wer and wif were used to refer specifically to "a male person" and "a female person," respectively. Subsequently, man replaced wer as the normal term for "a male person," but at the same time the older sense "a human being" remained in use. In the second half of the 20th century, the generic use of man to refer to "human beings in general" (as in reptiles were here long before man appeared on the earth) became problematic; the use is now often regarded as sexist or old-fashioned. In some contexts, terms such as the human race or humankind may be used instead of man or mankind. Certain fixed phrases and sayings, such as time and tide wait for no man can be easily rephrased (e.g., time and tide wait for no one). Alternatives for other related terms exist as well: the noun manpower, for example, can usually be replaced with staff or crew, and in most cases, the verbal form to man can be expressed as to staff or to operate.