Just adding a bit to what the others have said, you could almost think of the "parts" in a CAD program as being akin to the templates in a header file for a C library, i.e., they provide a symbolic representation plus an interface definition that are joined together.
In the case of electronic CAD, a "part" is what the part looks like in the schematic diagram, typically a box with numbered and labeled connections, and a representation of its physical dimensions and the location of its connections. Somebody has made sure that the physical details are correct, and that pin 23 on the schematic is pin 23 on the physical board.
Creating these parts is kind of a form of costly drudge work for designers, and errors in those parts can be hard to correct when your prototype doesn't work for some mysterious reason.
In the case of electronic CAD, a "part" is what the part looks like in the schematic diagram, typically a box with numbered and labeled connections, and a representation of its physical dimensions and the location of its connections. Somebody has made sure that the physical details are correct, and that pin 23 on the schematic is pin 23 on the physical board.
Creating these parts is kind of a form of costly drudge work for designers, and errors in those parts can be hard to correct when your prototype doesn't work for some mysterious reason.