Trifles falls under several classifications. First, it is a dramatic play, denoted by the spare description and the reliance on dialogue and audience interpretation instead of authorial exposition. Secondly, it is an example of feminist drama (Wikipedia) because of its focus on the issues and problems of women in society during the time of writing. The play shows how women are marginalized and ignored by the patriacial society, while in fact having a better understanding of empathy and societal influence than do men.
Another important classification is suspense; the play works through a generally peaceful investigation of a murder scene, but through clues and dialogue the true scope of the murder becomes more apparent and even scary:
MRS. PETERS: It was an awful thing was done in this house that night, Mrs. Hale. Killing a man while he slept, slipping a rope around his neck that choked the life out of him.
MRS. HALE: [Her own feeling not interrupted.]If there'd been years and years of nothing, then a bird to sing to you, it would be awful -- still, after the bird was still.
(Glaspell, Trifles, etext.virginia.edu)
The slow reveal of details and the slow realization of exactly what happened -- and more importantly, how long events moved to motivate the murder -- are essential in creating suspense in the story. The women are especially horrified, both by the actual depth of the murder and by the implications that something could have been done to stop it. The men, of course, never quite figure it out.