In Autocad you work mainly with 3 file types:
Autocad template file - essentially there's 2 main templates in Autocad, one for Imperial Units (Inches & Feet) and one for Metric Units (mm, cm, m). The one you should always use is "acadlt.dwt". I will include more information regarding the template files in a separate lecture.
Autocad drawing file - when you save you drawing, you will be saving in DWG format which stands for "drawing".
digital document file - basically, when you print in Autocad, it prints in PDF format.
There's one additional file format that you will notice when working with Autocad, which is the .bak file.
You will see .bak files beside your drawing files - they are basically backup files that gets created everytime you save your file.