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.

Monolayer is a sinle atomic thick layer may be 5-10 nm. Thin film is in the range of thickness having in the micron scale range....

Monolayers also tend to be covalently attached onto the surface

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.