What does it mean to be human? People have discussed and argued about this for a long time. It is of general interest, but it also plays a significant part in shaping your understanding of other significant issues. The question resolves into two inter-related parts, nature and identity: 'What does it mean to be human?' and 'What does it mean to be me?'
What does it mean to be human?
In Western thought, there are two traditional ways of understanding what it means to be human. They are associated with two philosophical traditions we find in the ancient world, in the Greek and Hebrew cultures. This does not mean, of course, that every Greek person had one understanding and every Hebrew person had the other; neither does it mean that these two understandings were always clearly distinguished. But for all the borrowings and overlaps we can identify in particular individuals and groups, the basic concepts which were passed on to subsequent generations and cultures from these two traditions can be clearly distinguished. There is also a non-Western tradition which has gained some ground since the Enlightenment
According to the Greek tradition, a human is a soul trapped in a body - an enfleshed soul, if you like. We can ignore the question of the difference between soul and spirit here: the issue is purely about the relationship between the material aspect of the human and the non-material aspect. And the key point is that what is truly human is our soul; the flesh is what we have in common with the other animals. The soul is distinct from the body, which raises the question: what happens to the soul when the body dies? The Greeks thought this question was important, which is why there are so many inventive Greek stories about what happens to people in the afterlife, especially the individuals who anger the gods.
This belief leads to two basic options: after death, either the soul moves on to some ethereal reality (or, more often, one of a number of ethereal realities, providing a variety of experiences, from the very good to the very bad), or the soul returns to this world through reincarnation. But it is important to recognize that reincarnation is a process which happens to the soul (the soul material, if you like), not to the person: your soul may inhabit a new body, but it is not you, it does not have your memories or personality, it does not have your hopes and dreams, your likes and dislikes. Reincarnation is rather like the physical process of decomposition: worms may eat your body and ducks feed on the worms, and your flesh may become new flesh, but that new flesh is in no sense you any more.
According to the Hebrew tradition, a human is an animated body. Again, the distinction between soul and spirit is essentially unimportant: what matters is that, for a period of time, something turns a chunk of matter into a living being. Humans, like the other animals, are animated matter; they are like the other animals in many ways, but they are unlike the other animals because they have a different relationship with the creator God. Humans are chosen by God, but being chosen does not mean we are better or superior: it means we have been given a task - two tasks, actually: to take care of the rest of creation, and to show the rest of creation what God is like. There are no stories about what happens to people after they die, because people who die lie in the grave and don't do anything. But there is a small occasional hint in the Hebrew writings that people who are remembered by God have not completely disappeared (just as many people say today of those who have died: they live on in our memory), and perhaps through God's creative power they might be given life again. This small hint is picked up in the Christian development of the Hebrew tradition, which holds out the hope of a future resurrection into a better physical body, living in a better physical world.
There is a third possibility: that being human means nothing. In this view, which is the official belief of most modern Western societies, but is actually believed by very few people within those societies, there is nothing special about humans - they are nothing more than advanced animals, and life itself is just a cosmic accident, an unlikely arrangement of atoms which is quite likely to occur somewhere because the universe is so vast.
From this perspective, we are simply a collection of atoms which is constantly changing; what it means to be human is a meaningless question: 'human' is just a word we use to describe a group of organisms which are able to interbreed at this point in time, but has no clear origin to distinguish it from other hominids or other primates or other mammals.
This is essentially the perspective of some non-Western religions: they teach that what we experience and what we believe ourselves to be are simply illusions, which we need to free ourselves from.
What does it mean to be me?
One key point is that I am not just my brain (see: You Are Not Your Brain for more on this). Apart from anything else, the brain is deeply connected with the rest of the body. Scientists are increasingly understanding that a large part of what makes me 'me' is tied up with physical realities, such as the functioning of the ecosystem in my guts. As this New Scientist article explains, Consciousness isn't just the brain.
Another key point is that I do not just exist in the present: if you stole from me yesterday, you stole from me, not some 'past version' of me; I put money into my pension so that I will have enough to live on - not so that some 'future version of me' will have enough to live on. The lived reality is that I am not only my present self but also, somehow, my past and future selves.