In building a new home my blueprints show the need for #12 2 conductor and #12 3 conductor romex. What would I use each for?

12-2 (usually labelled 12-2 + Ground) is 12-gauge, two-conductors: black/hot and white/neutral, plus a bare copper ground. It is used for any ordinary 120 v circuit such as receptacles and lighting.

12-3 is all of the above plus a third red wire, generally used for circuits involving 3-way switches, where hot is allowed to take either of two pathways, black or red.

12 gauge wiring is normally used with 20 Amp single pole circuit breakers.

Hope that helps. And welcome to AnswerPool!

Thanks Professor both for the info and the welcome.