AutoCAD is a Computer-Aided Design (CAD) software suite developed by Autodesk that is commonly used by various Engineering design teams to easily create models and sketches of their projects. AutoCAD can be used to create 2D drawings and sketches (e.g. floor plans, cross-sections, etc.) and 3D models. Autodesk has been revising and improving AutoCAD for many years; the first version was released in 1982!
AutoCAD is quite easy to create 2D designs and sketches and has a very easy learning curve, making it great for drafting purposes. However, there is a slight learning curve to creating 3D designs (and some very complicated 2D designs). Luckily Autodesk provide numerous tutorials and educational resources, along with a pretty strong external community.
As expected with any CAD software package, purchasing a commercial licence (so that you can use it in industry or as a consultant) is quite expensive. Despite this, University students (provided you have a valid student email from a registered University) can access a free student version which can only be used for education purposes. If you do not have a valid student email, Autodesk also offers a free 30 day trial.