Computer Aided Drafting (CAD) means using the computer, instead of the classical tools (pencil, ink, rulers, paper) to create drawings. There are several advantages to this, for example the drawing can be subdivided in smaller parts, that can be reused or be worked on by several architects, updating the drawing is much faster than with hand-made drawings (where you often needed to redraw the whole drawing), and several tools can help you to check your drawing for errors. Besides this, computer generally allows you to work in real-world units, and does the scaling automatically, so your drawing fits on the printer sheet.
CAD Applications are nowadays complex and very carefully conceived. Therefore most of them are highly expensive software. Among the most used worldwide, are Autocad, Archicad, Inventor, Microstation, Vectorworks or Allplan. Almost all engineering and architecture offices on the planet use one of them, so knowing at least some of them is very important in the professional world, and usually the most important requirement on architecture job offerings.