Near-future science-fiction is science-fiction that takes place in the near future. That last sentence being a tautology, I will try to expand on what falls under the nebulous cloud of the "near future".
Near-future science-fiction is not a separate genre, but rather a term for any science-fiction marked by relatively slight differences from the present day. This can mean it is close in chronological time to the time of writing, or just that it takes place on a world not that much different then the time of writing. Elements of plot and theme also determine whether it is near-future. If there is interstellar travel or a massive alien invasion, it is probably not near-future. In fact, many near-future science fiction stories have no space travel in them at all.
Near-future science fiction also seems to be more oriented towards sociological themes, and is more likely to address the impact of a technological change on society. Because of this, it is often considered to be more of "serious" fiction than other science-fiction. For example, 1984 is usually not treated primarily as a science-fiction novel.
Of course, with such a hazy definition, there are very few definitive rules about the purposes and styles of near-future science-fiction.
A list of some famous works of near-future science-fiction works includes:
There are many other examples that could be given, but just from this list, the variety of themes and styles present is easy enough to see.