We are planning a trip to Spain in December '14 for 10 days starting in California. Unsure which part of Spain to visit but do not want to rush through the entire Country in 10 days. Would like to cover popular tourist attractions and take road/train trips. Also wondering if some parts of Spain (south?) would be warmer in December than others?
Thanks for any ideas/resources for us to explore.

Yes, southern Spain will be quite a bit warmer as long as you stay out of the mountains. Madrid is on a high plateau in the middle of the country and has a real winter - often with snow. But you can't see the whole country in so little time anyway - so you may want to stick to the south.

As noted, pick a spot and enjoy it. Sevilla isn't all that bad in December, but I'd go for early December rather than later in the month unless you want to be there for Christmas, which is celebrated (throughout Spain) from the 24th of December to the 6th of January.

Weather is totally unpredictable even weather history websites cannot foretell what going to happen.
The mountains are a great skiing area which it is said you can be on the beach within an hour, but it it certainly will not be sun bathing time.
If sun and warmth is important then maybe head for the southern hemisphere where it would be summer at that time of year.

I too would recommend sticking to Andalucia and the South, it will be a lot warmer.

In 10 days, you can see Granada, the Costa del Sol resorts, the White Villages and Seville and Cordoba. Plenty there to fill up your time, maybe too much.

A good guidebook or two should prove invaluable to you.