DWG
Short answer
Autodesk's proprietary binary drawing format — the file that contains every AutoCAD or Civil 3D drawing.
DWG is Autodesk's binary CAD drawing format, used by AutoCAD, Civil 3D, MEP, Architecture, and Mechanical. The format is versioned by AutoCAD release (e.g. AC1027 = AutoCAD 2013), and saving to a newer version means older releases can't open the file. The open-source Open Design Alliance (ODA) provides DWG read/write libraries — Autodesk's own DWG TrueView is free for viewing only. Plugins that batch-process drawings typically use ObjectDBX or the ODA Drawings SDK.