Winter Cap
A warm hat that covers your ears is a must on cold days, especially windy ones, and can be one of the best steps you can take to keep warm. Merino wool, fleece, and technical-fiber options are available at specialty running stores.

Light Jacket
A jacket, usually a polyester blend, serves to keep you warm, keep off wind, rain, and snow, and manage your perspiration. It's an essential piece of equipment on cold, windy and/or rainy days. Use the zipper as a "thermostat" – zip up or down on the run, as needed, to stay comfortable.

Long-Sleeve Shirt
A long-sleeve shirt made of high-tech polyester will pull moisture away from your skin, keeping you from getting clammy and cold on a cooler day.

Sports Bra
For women, a sports bra is a "must-have" piece of equipment for running happy. If you are experiencing pain and swelling, you are most likely in the wrong bra.

Gloves
When in doubt, wear gloves or mittens; if you get too warm, they're easy to tuck into your shorts or tights. Gloves keep your hands warm on a cold day, but mittens are an even warmer option when temperatures plunge below freezing.

Tights
The first level of insulation for your legs. "Classic" tights are generally a polyester and spandex blend. Looser running pants (also stretchy, but not as form-fitting as tights) are another option here. In extreme cold, tights under pants is a good layering strategy. Underwear under the tights is a good idea; just try to avoid cotton.

Running Shoes
Choose a pair of running shoes with the cushioning and stability appropriate for your biomechanics. Or go barefoot (weather permitting, of course). If wearing shoes, a good pair of moisture-wicking running socks can help prevent blisters.

http://www.runnersworld.com/what-to-wear

Winter Cap
A warm hat that covers your ears is a must on cold days, especially windy ones, and can be one of the best steps you can take to keep warm. Merino wool, fleece, and technical-fiber options are available at specialty running stores.

You can use a winter cap, but unless it is really windy then a normal ball cap with ear coverings will work too. Something like this: Keep it water wickering, because you can still sweat in 20 degree weather, believe it or not.

Light Jacket
A jacket, usually a polyester blend, serves to keep you warm, keep off wind, rain, and snow, and manage your perspiration. It's an essential piece of equipment on cold, windy and/or rainy days. Use the zipper as a "thermostat" – zip up or down on the run, as needed, to stay comfortable.

I personally opt for a nice vest. Jackets tend to make my body over heat and I like the fact that the cool air can help cool my core if I get too hot in all my layers.

Long-Sleeve Shirt
A long-sleeve shirt made of high-tech polyester will pull moisture away from your skin, keeping you from getting clammy and cold on a cooler day.

Look for the base layer types of clothing...like long sleeves underarmor, or brooks long sleeve running technicals.

Gloves
When in doubt, wear gloves or mittens; if you get too warm, they're easy to tuck into your shorts or tights. Gloves keep your hands warm on a cold day, but mittens are an even warmer option when temperatures plunge below freezing.

Lighter mittens for 20 degrees will be fine. The heavier they get the clamier your hands get. Save the heavy mittens for extremely cold days.

Tights
The first level of insulation for your legs. "Classic" tights are generally a polyester and spandex blend. Looser running pants (also stretchy, but not as form-fitting as tights) are another option here. In extreme cold, tights under pants is a good layering strategy. Underwear under the tights is a good idea; just try to avoid cotton.

Yes! The Nike/Asics/Adidas/Etc...work great. But as your running gets better and you become more involved in the sport start to invest in the Brooks/North Face/CW-X brands as they have great quality and the clothing is specifically designed with runners in mind.

Running Shoes
Choose a pair of running shoes with the cushioning and stability appropriate for your biomechanics. Or go barefoot (weather permitting, of course). If wearing shoes, a good pair of moisture-wicking running socks can help prevent blisters.

Balega socks! These are great socks!

As for the extra layers over your tights...that is another good thing about investing in the Brooks/North Face/CW-X brands, you don't need to hide your booty because the material is in no way shape or form see through. I have found some Nike/Asic/Adidas tights to actually be see through, so I have to throw shorts over them. If you are one of those people who do not tolerate cold very well at all, a pair of light weight sweats or wind breaker pants over the tights can help lock in body heat and reduce wind chill.

The general rule my running club uses for 20 degrees weather is 2 tops (one long sleeve/one vest), one bottom (tights), light weight mittens and something to cover your ears.

It was in the 20's when I ran the Gobble Jog on Thanksgiving:

I wore tights with shorts (keeps my hips and thighs warm). Feetures socks. A wicking singlet with the 2007 Gobble Jog race shirt (cotton) over it to show my dedication to the race. Gloves. A neck gaiter. A wicking Nike skullcap. I was chilly but not freezing waiting to start (which is ideal). After a mile, I was perfectly comfortable.

I love that headband. I'm currently lusting after this though:

http://www.oiselle.com/shop/running-tops/flyte-long-sleeve

Thumbholes!!!