The definition of drama is that it is a literary work that is intended to be presented on a stage or--in contemporary times--in a film by actors to an audience and that has characters who are in some sort of conflict that involves some sort of action and a crisis with a resolution all occurring within a specified atmosphere. The conflict may be physical action, as in Shakespeare's Henry the IV, or mental action, as in John Osborne's Look Back in Anger, where they essentially debated philosophical and moral issues. A crisis arises that is in keeping with the kind of action presented and that must be resolved by the end of the drama. There are subcategories under the main category of drama: there are comedies, tragedies and tragicomdies, with further subdivisions such as farce and satire or comedies of manners and melodrama.
It seems a little more difficult to identify the "nature of drama" as some would say its nature is identified by its definition. In other words, some may say the nature of drama is that it is a literary form presented by actors, as opposed to a form presented between the covers of a book, that presents action that leads to a crisis that is resolved (or not in absurdism). Perhaps, though a bit more detail can be identified that might elaborate upon the nature of drama.
The applicable definition of nature in this usage is that the nature of a thing is its essential parts; its peculiar qualities. By this definition, the most distinguishing essential part is that already mentioned: actors who stage the story for the audience to see in company with a larger group of people that replace pages that contain the story for the audience to privately read. Another essential part to distinguish drama from other literary forms is that character's personas and dialogue and actions must provide the sole information about the drama--unless there is scripted a narratorial character who comments upon the other actors' action and dialogue, as in Thorton Wilder's Our Town and Williams' The Glass Menagerie. Therefore, the essential nature of drama is to present the action, conflict, crisis and resolution of a story through character actors before a group comprising the audience in a public forum where mental and emotional (sometimes physical) reactions are a collective experience.