In an emotional sense, love. 🙂 In a scientific sense, we’re all members of one species. This has to do with our DNA. When two humans meet, they can produce fertile offspring. Humans are the only members of the genus Homo (our scientific name is Homo sapiens) still around. There used to be neanderthals (Homo neanderthalensis) as well, but our ancestors a long time ago probably murdered them.
Wow, Maureen has explained this really well! The only thing I would add is that being human is characterised by walking upright, being able to use heavy and complex tools, we have very complex ways of communication compared to other animals and we tend to have larger and far more complicated brains! This is a great page about Humans https://en.wikipedia.org/wiki/Human