This is an helpful resources page that you can always come to for all your 3D printing needs. This page will improve with time.
3D models repository
Here are some sites where you can easily find 3D models to print.
CAD is the acronym for Computer Aided Design software for drawing 3D models. Virtually all CAD software can export 3D models in stl format. There are CAD software for all budgets, pro and free.
Free or open source:
The slicer is the software that will generate the 3D printer instructions (.gcode) from a digital 3D model (STL file). It cuts the model into thin horizontal layers which are then treated like 2D drawing one above the other. The slicer is a Computer Aided Manufacturing (CAM) software that will generate the toolpath for the 3D printer to complete the part.
The host software can control the 3D printer. This software sends the G code commands from the computer to the printer
Firmware is the software resident on the 3D printer motherboard. It interprets G code commands into stepper motor movements. The firmware must be configured for each 3D printer model.