Monolayer is a single molecule thick, and usually orered in a tightly packed plane. Think of it like a lawn of fresh grass. A thin film can be multiple molecules thick, and doesn't have to necessarily be a tightly packed arrangement. All monolayers are thin films but not all thin films are monolayers.
Langmuir is used for time that needed to cover one single monolayer of a surface under sticking conditions (assume sticking coefficient is 1.00) in UHV systems.