Long Day's Journey into Night is part of a genre in theater called Realism. This is not, it should be said, the same as literary Realism. In theater, Realism was a long movement beginning in the 1870s and continuing through the 20th century. It is more of a method than a literary movement: in realist theater, people speak and act as they would in real life, using unadorned, prose language. The goal is to create a facsimile of reality, and the conflict in the realist theater is usually psychological. In literature, on the other hand, Realism is a more specific movement from the second half of the 19th century in Europe and the United States. A reaction to Romanticism, Realism had a particular focus on urban life, industrialism, poverty, and the working class.
Long Day's Journey into Night, with this definition in mind, is certainly a realist play. The conflict is psychological, and the mental states of Mary and Edmund, in particular, are at the center of the play. All of the dialogue is commonplace and relatively unadorned by poetic turns of phrase. The exceptions to this—that is, when the dialogue feels less "real"—include the great number of literary allusions by James, Jamie, and Edmund, and Mary's more poetic monologues in Acts Three and Four. But these less realist moments are explained by the plot, with James's intention to teach his sons to act and read as he did, and with Mary's loosening mental state as the play continues. Likewise, all lights, sounds, and music (according to the stage directions) are "diegetic," meaning that they are meant to replicate what would have been really heard and seen in the house. The most widespread instance of this is the foghorn, which echoes quietly in the theater, just as it would have been heard in the cottage.
In terms of its theatrical form, Long Day's Journey into Night is a tragedy. Literary critics generally divide theatrical tragedies into two large categories: Elizabethan tragedies, like those written by Shakespeare and his contemporaries under the rule of Queen Elizabeth I; and Greek tragedy, like those written in ancient Athens by Sophocles and others. This play is an Elizabethan tragedy, usually characterized by great men brought down by long-held features of their character which, because of a change in circumstance, become fatal flaws. Tyrone fits the bill well of a Shakespearean tragic hero: his miserliness and high expectations of his family lead everyone to ruin.