The enotes Study Guide on Gothic fiction says the following:
Gothic literature, a movement that focused on ruin, decay, death, terror, and chaos, and privileged irrationality and passion over rationality and reason, grew in response to the historical, sociological, psychological, and political contexts of the late eighteenth and early nineteenth centuries.
Gothic literature is, one could say, a form of Romanticism. Certainly, without Romanticism, Gothic fiction wouldn't have flourished and become popular when it did. Examples of Gothic fiction include Mary Shelley's Frankenstein from England, The Phantom of the Opera from France, and pretty much anything by Poe in America.
In addition to the details above from enotes, Gothic fiction usually involves ghosts or some form of the supernatural, castles or old mansions, and an eerie tone or mood.
Realistic fiction is just about the opposite of Gothic. It attempts to portray reality as best as it can. It stresses concrete, specific details (verisimilitude) in an attempt to appear realistic. It also deals with life-like characters in realistic settings and circumstances.
Gothic fiction generally combines equal parts of horror and romance to create its own specific literary genre. Settings usually include an old house or even a castle; architecture of the Gothic Revival originally played a part in the creation of the style. An atmosphere of terror usually plays an important part in the story line, and a love interest or even a family is nearly always included. English writer Henry Walpole is often considered the first author to use this style; Edgar Allan Poe popularized it in America a half century later. Stephen King's novels are certainly among the leading examples of this genre today. Since gothic fiction often adds a supernatural element, this is one of the primary differences between it and more realistic fiction, which stresses true-to-life situations and problems.