There is an old suspicion that the past and the future are not real: the only reality is the present. What has been is no longer; what will be is not yet. Over the centuries, there have been many different formulations of this view (in twentieth-century philosophy, it's often called 'presentism'); and I have myself given a version of it earlier when I counted the past and the future among the forms of unreality.
The view that only the present is real has its roots in some familiar common sense observations. People and objects cease to exist: people can die, objects can be destroyed. We have all been witness to processes of that sort. And likewise, we also have seen humans being born and growing into persons, objects being produced. Thus it's a fact of everyday life that many people and things we can refer to are only accessible from memories, records, by inferences from past actions and so on; something similar holds for future people. (You can talk about your great-grandfather or about your great-grandchildren, and it's perfectly clear that they are past and future respectively, and you'll never encounter them directly.)
Of course, this denies only some reality to the past or future. In some sense, your great-grandfather is still more real than, say, a character in a novel will ever be. Even if you know next to nothing about him, there is still some impact he has made: at the very least in having a part in the events that brought about your own existence. And yet, the more you think about it, the more difficult it seems to grasp exactly where the difference lies. For a start, fictional characters also can change the world in many ways. Children may be named after them, people may find inspiration in them, or they might become an element of the popular imagination (genres as different as the James Bond movies or Joyce's Ulysses have had that effect, as can be read off clearly from the teenage girls in the audience of a Connery-Bond screening or the crowds at the Bloomsday parties every year). If there is a difference between a person who really existed in the past and a character in a novel who never existed, it can't simply lie in the influence that (past or fictional) person had on the world.
There is such a difference for present people who really exist and fictional people: an existing person you can encounter, talk to them, shake their hands, interact with them in many ways; you can never do that with a fictional character. (You can shake Goofy's hand at Disneyland; but that statement is only true in a loose sense of speaking: we all know that it's an actor playing a role, and anyway you can have an actor play your great-grandfather's part at a family party and shake his hand, too.) But there is no such difference between past people and fictional people.
You could say that there is no such difference any more, and perhaps that would be a clue to a better candidate for the difference we're looking for: for people who once existed there has been a time when you actually could interact with them, while there has never been, and never will be, such a time with respect to a fictional character. Even this line of thought needs a lot of refinement, though. It works only partially. For instance, many of us have known their own grandfather, some have perhaps even known their own great-grandfather, talked to them, interacted with them. But once you get to a generation sufficiently removed in the parental chain, there is no more overlap period where you could have a direct interaction with your ancestors. To make the account work fully, it has to include some transitivity: a whole chain of potential direct interactions would connect you to a person who existed, while there is no such chain that can connect you with a fictional person. Take an example.
You can't have a conversation with Jane Austen; your mother couldn't, either; but your great-great-...-grandmother of some degree just might have — she didn't live in England maybe, but if she had traveled there and if she had done this and that, it would have been possible. And you're connected with that interaction via all the links between you and your mother, her and your grandmother, and so on. In that sense, there is a chain of potential direct interactions (they don't have to have happened in actuality) between you and Jane Austen, who, after all, really existed. There's no such chain between you and Elizabeth Bennet, whatever your great-great-...-grandmother of some degree would have tried. Elizabeth Bennet never existed. She's fictional.
But then again, notice that we had to make heavy use of the notion of possibility in this account. Now think about this: you don't have an older brother (if you have, suppose you haven't), but you can easily imagine what the world would be like if you had one; think about the interactions you'd have with your possible brother. Of course, since that brother doesn't exist, we're talking about an unreal person. However, in terms of the possibilities we had to invoke, he seems to be much closer than Jane Austen was. And she did exist. So where exactly does that leave our account of the reality of persons who existed in the past in terms of possible interactions?
If you've made it here, you're probably beginning to sense that there are interesting things to explore about the reality (if any) of the past and the future. They have been, of course, extensively discussed in the philosophical literature over the past twenty-five centuries or so, and there's a lot of fascinating things to learn from all this for the philosophy of unreality. Stay tuned...