Computer Aided Design (CAD; properly called Computer Aided Draughting) is a facility for producing and storing drawings on a computer.
Much of the material on this page is taken from my experiences with AutoCAD. However, the principles, and possibly at least some of the functions, apply to any CAD package.
The drawing itself is done in model space, and the border in paper space. The model space drawing shows up in a viewport when one is added to the paper space sheet, and the view can be manipulated. A CAD drawing can have more than one paper space sheet, which is why the drawing border includes a space for the sheet number as well as one for the drawing number. The sheet number is normally given in the form "X of Y", where Y is the number of paper space sheets in the drawing file; presumably, this is to ensure that anyone who receives a printed copy knows whether they have everything in a particular file.
CAD drawings can be produced faster and more accurately than paper ones. One reason for this is that the user can select points, for example the point at which a line is to start, by typing coordinates rather than by clicking the mouse. If the symbol "@" is inserted first, the coordinates denote the location of one point relative to the previous point, so the coordinates can denote such things as the length of a line or the dimensions of a rectangle.
Another feature that improves the speed and accuracy of CAD drawings is the object snap function. The user can select types of points, such as the end of a line or the centre of a circle. When the cursor is moved close to a point of a selected type, a symbol appears at that point. On the next click of the mouse, the computer acts as if the cursor was exactly at that point. This eliminates the need for the user to zoom in on part of a drawing.
Symbols that are used more than once can be stored on the computer and copied into drawings as and when needed. The symbols can be copied and pasted, inserted as blocks, or inserted using the external reference function. Blocks cannot be amended, while if the external reference function is used, changes to the original symbol are automatically copied to the duplicates, although the duplicates must be on the same network as the original symbol.
Just as CAD drawings can be produced more quickly than paper ones, so they can be amended more quickly. This means mistakes are easily corrected, and a drawing is easily modified if a design is updated.
Another advantage of CAD is that CAD files are easier to store than a set of full-sized drawings. The drawings can be kept on the computer and printed as and when a paper copy is needed.