Period (or historical) dramas are Drama films that are based around historical events, people, places, and/or things. Whilst some come in the form of pseudo-factual Docudrama films, many simply follow traditional drama film-making practices within a historical setting.
While generally period dramas are often lax with the authenticity of their historical content, they tend to stay as true to possible as the period they're representing. Film-makers use very specific costumes, make-up, sets, props, and even speech dialects to create an authentic representation of a story set within such a time period. When focusing on the lives of specific individuals, these films are referred to as a Biopic.