[Abominable No-Man]

What Maser said is pretty close.

In a nutshell, a select fire rifle has some extra/different internal parts. Depending on the weapon in question, they will vary in nomenclature/layout/etc. A select fire M16, for example, has automatic sear and disconnector assemblies inside the lower receiver, along with the associated pins, springs, and selector switch. (Alsp, some parts, like the bolt carrier of a weapon, may or may not affect select fire capability, but are considered select fire parts as well. Be careful about what you install.)

The parts are not "drop-in": depending on the weapon in question, a different receiver template is required in order to drill the extra hole/s required in order to correctly install the parts. This is not something that is advisable to try with your Black and Decker cordless drill; it requires a high level of precision, and is illegal without a class 3 manufacturer's license.