They can both be used for wiring circuits, the wire gauge that's used is dependent on the circuit breaker that protects that wire. 14 gauge wire is protected by a 15 amp breaker, and 12 gauge wire is protected by a 20 amp breaker. It's ok to use 12 gauge wire on a 15 amp circuit but I don't advise it, You really want to maintain a consistancy of using the appropriate wire for the proper breaker and not mix the two, if done incorrectly it can lead to a fire hazzard. Here's a link that'll explain a bit more in detail for you.
If this is a 15 amp breaker then stick with 14 gauge wire. It would also be a good idea if you bought your self a good book to learn about home wiring. The Black and Decker book on home wiring is a good one and will help tremedously in your projects. You can find one at Home Depot or any good book store.
I used to wire houses with two gauges of cable 14 and 12. Now a days it doesn't offset the cost very much to use the same cable throughout - so 12 is the norm for everything. Ideally you could use 20 amp breakers on everything but that's up to you, no harm in putting a 12 ga line onto a 15 amp breaker. The cost of a 15 - 20 amp breaker is the same. Just mind you that circuits in a bedroom HAVE to be an "arc-fault" breaker under the new code rules.
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