There are several steps to taking a idea and creating a object. Each step has several methods.
These are some examples:
Download thing -> CAM -> "Print" -> "thing" post work
"idea" -> CAD - > CAM - > "Print" -> Casting -> "thing" post work
Scan -> CAM -> "Print" -> "thing" post work
Line Art -> 2.5D software -> CAM -> Mill --> "thing" post work
"idea" -> CAD -> Printing service
Download thing: 3D objects are availible from public and private archives like Thingiverse , GrabCAD , and for pay sites.
Scan: A process that uses lasers, photographs and software to collect 3D information about existing objects - Catch123D,
CAD: computer aided design software
CAM: This a general classification of software used to turn a STL 3D object file into information the mill or printer can understand
Print: This is the action converting information into a solid object by additive (FDM, Light polymerised, or subtractive (milling, laser cutting, plasma cut) processes.
Printing service: Print for hire, these services have abilities beyond the average hobby printers - Shapeways, Ponoko, Pololu
Post work: Removal of printing artifacts such as defects, support material, refining surface finish, surface sealing, painting, etc.
Casting: This is the process of using materials that change from liquid to solid to replicate the surface of another object - plaster, smooth-on, oogoo, hotmelt, aluminium, resin, glass.
Some of the processes can be skipped depending on the desired end result.
Nearly all methods require some CAM (slicer, gcode generation, etc.). The CAM software depends on the "print" machine.
3D printing directly to ABS or PLA plastic is often the end of the process.
Lost foam casting and Lost PLA casting are useful for Aluminium, brass, and bronze casting.
Casting in roto-molds are useful for resin plastics.
Plaster, Ceramic slip and Ceramic Drap molds are useful for glass molds and ceramics.
Lost PLA - Lost PLA example
Lost Foam - High Temperature Casting
Resin casting Gears!