Science fiction, as a genre, is characterized by a one or more of the following elements: setting in the future, outer space, or both; heightened technology; presence of aliens; scientific ideas and elements that may sound true or realistic but do not actually exist.
The Host, by Stephanie Meyer, is set in the future. Aliens have come to occupy or embody humans by attaching their "souls" to the human brain and taking over the body (this is where the title The Host applies).
This novel blends these somewhat basic science fiction elements with an even stronger romantic tone. The story itself, though based on science fiction principles, is more of a human interest and love story. This is one thing that Stephanie Meyer does particularly well. Many readers who would not claim to love science fiction and fantasy find themselves reading and enjoying Stephanie Meyer's novels.