A geometry definition file. CAD models are exported as OBJ files then imported into a slicer program. The slicer program then converts the file into G-code to be interpreted by the 3D printer. Similar to .STL files.
In 3D printing offset refers to layers that are not printed directly inline with one another and are instead shifted to the side. This is often a printer calibration issue and will impact the quality of a print.
It is an outer layer on the edges of a model. Models can be printed also on outlines without infill. The number of outlines can be adjusted by the used slicing software.
Overhangs are geometric shapes in a 3D model that extends outwards and beyond the previous layer. Overhangs have no direct support on it so it is difficult to be printed. They require supports to be printed.