CAD, or computer-aided design (CAO in French), covers the software used to design and describe a part or an assembly: 3D solid models, assemblies, dimensioned drawings. The result, the digital model of the part, feeds both the manufacturing drawing and the CAM software.
Most software models parametrically: the designer draws a sketch, applies features to it (extrusion, revolve, fillet, hole), and the dimensions drive the shape; changing a dimension updates the whole model. Parts are then assembled with constraints, which makes it possible to check for interference and generate a bill of materials. The drawing produces the views, sections and dimensions, with tolerances. SolidWorks, Inventor, CATIA, Fusion or FreeCAD are examples. To exchange files with a workshop, you export in a neutral format: STEP (STEP file) for 3D, DXF for cutting contours, STL for 3D printing, PDF for the dimensioned drawing. The guide to file formats compares them.
CAD is the starting point of every project: it lets you simulate, check the overall size, calculate a mass (see the mass calculator) and prepare for manufacturing. Companies without a design department can rely on an external design office to do it.
In a quote request. Attach the 3D model in STEP and the dimensioned drawing as a PDF. The model alone is not enough: it does not always carry the tolerances, the material, the surface finishes or the treatments. If the model and the drawing disagree, state which one prevails, generally the drawing for toleranced dimensions.