Trash

Films characterized by rock-bottom technical and artistic quality, including amateurish production values, mediocre casts (primarily dilettantes, often family and friends of the director), lousy scripts, and even worse editing.
Once an incompetent director takes the reigns, things tend to go downhill fast, all too frequently resulting in shaky shots, out of focus scenes, incomprehensible audio, either overexposed or practically pitch-black photography, shoddy costumes and sets, and bloopers galore.

In a well-known Italian essay on the trash phenomenon ("Andy Warhol Era un Coatto: Vivere e capire il Trash" - 1994), novelist/journalist Tommaso Labranca maintains that "Andy Warhol was Trashy" and lays out what he sees as the fundamental elements of the "trash" genre. Two such elements - "botched imitation" and "incongruity" - frequently lead to massive doses of unintentional humor when it comes to filmmaking, which at least partially explains why trash films have found a decent fan base over the years. In fact, today many Z movies have become genuine cult classics or even "risen" to the ranks of "So Bad It's Good".

Among serial purveyors of trash films, some stand-outs include Edward D. Wood Jr., Al Adamson, Andy Milligan, and Don Dohler, to name a few. (Italian cinema has also seen its share of trash, thanks to the likes of Renato Polselli, Claudio Fragasso, and Andrea Bianchi, among others).

P.S. The eye-popping prevalence of trashy elements in Troma films - as well as works from other particularly eccentric filmmakers such as John Waters - earns them the "honor" of this Tag, despite the fact that they're a far cry from the classic low-budget Z movie, especially when you take into account Troma's biting social critique, their deliberately deranged humor, and (notably with Lloyd Kaufman in the director's chair and James Gunn as scriptwriter) their technical competence and high-quality content.