Romance, Adventure
The title by which we know Béroul's version of the popular medieval legend of Tristan and Isolde, The Romance of Tristan, marks the story as part of the medieval romance genre of literature. Medieval romance was a genre that became popular among aristocrats beginning in the 12th century. These romance stories, usually written in verse, told of the adventures of a knight-errant as he sought to prove himself and define his identity against other knights, fantastic beasts, and evil villains. They often incorporated magic and the supernatural as an important part of the plot.
Medieval romances might also spend some time on the relationship between the knight and his lady, but this was usually a minor part of the story. That's where The Romance of Tristan differs from your typical medieval romance: it spends most of its time on the relationship between the two lovers. Even when Tristan's engaging in typical knightly behaviors—jousting, fighting, disguising himself as someone else—he's doing so in order to carry on his adulterous affair with Yseut. This narrative focus makes The Romance of Tristan more like what we expect of a modern romance story than a medieval one.