There is a ton of information about The Metamorphosis in the enotes study guide about the novella, and I've selected specific sections in the links below to point you in the right direction--please use this information as an addition to the overview I'll give here!
The Metamorphosisis one of the most famous stories of all time. It is the piece for which Franz Kafka is best known, and it is considered representative of his work. It can be classified several ways, one way it can be reliably categorized is using the term Kafkaesque. This is a real term based on Kafka's one style that describes literature which includes, as the enotes overview puts it: "an unsettling, disorienting, nightmarish world that is at once both fearful and menacing in its ambiguity and complexity." The guide goes on to note that Kafka's "haunting, disturbing, and sometimes grotesque images, combined with his struggling but ultimately defeated heroes, defined an age wherein alienated man—the anti-hero—grappling with meaning and justice in an inscrutable world, is denied answers to both." That, in a nutshell is what we're looking at. Kafka defined his own literary movement, and The Metamorphosis was the main channel through which he did it!
You should also know some other interesting things about the story that make it notable in a literary sense. First, it has been highly debated for a long time by many prominent scholars. Is it a Freudian text? A Marxist one? An Existentialist one? Is it really just all about Kafka's uneasy relationship with his own father? Could it be a commentary on the historical oppression during Kafka's time? What could the ambiguous ending really mean? All of these questions have provided decades of debate for scholars. The text is ambiguous enough to support all and confirm none.
Another important thing to know is that the topic of metamorphosis (a change from one physical form to another) is an ancient and frequently used plot line for a story--one in which a character must prove his humanity after a transformation into a beastly form, and hopefully resolve it by returning to human form (unfortunately for Gregor, this is never achieved in Kafka's text). Think of Beast from Beauty and the Beast,the frog from the "The Frog Prince," Jekyll fromDr. Jekyll and Mr. Hyde, or even The Incredible Hulk. The metamorphosis plot is one of the big ones, and Kafka's version is one of the best known, most studied, and simply most haunting.