In general, there are three types of basic CAD files, they are:
1. Part
Part is the most basic entity of the CAD files. Part is a representation of a single design component and most of the time is the building block of a bigger system.
2. Assembly
Assembly is a combination of parts or another assemblies to form the whole system. An assembly inside an assembly is commonly called as sub-assembly.
3. Drawing
Drawing is a 2D drawing consisting details of either part or assembly. Drawing of a part usually consists of the dimension of the part, how to fabricate the part, etcc. Drawing of an assembly usually contain the information how to assembly the whole system.
To give an illustration of this structure, imagine of a simple pen like below.
The pen itself is an assembly, consisting some parts namely the cap, outer tube, end cap, rubber handle, and the refill. The refill is an sub assembly in this case as the refill consists of the tip, the refill tube, and ink.
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There are, however, some CAD system that has more than three types of CAD files as mentioned above. For example, Autodesk Inventor has Presentation type file, which can is mainly to create a "display" of an assembly, for example exploded view. These additional CAD files are just extra, and create some unique selling point for each CAD software. But on principal, all CAD software would have the three basic CAD files as mentioned above. Part, Assembly, and Drawing.
