Basically, there are 5 winter cycling essentials.
- A windproof jacket or vest
- Mid-weight top
- Leg covering of some kind
None of this has to be cycling-specific.
If you're going on a long ride, jackets (with sleeves) are best when it's truly cold (30s and low 40s). Vests are better closer to 50 degrees. If you're commuting around town (working up less of a sweat), a jacket will probably be more comfortable even at the warmer temperatures.
Gloves are warmest when windproof, but non-windproof will also do. Thinner gloves (like the stretchy knit ones you get for a $1 work fine for 50s and 60s. In the 40s, you definitely want something more insulated. The colder is it, the more important windproof becomes.
A beanie style hat is best to fit under helmet. Again, the colder it is the more insulation you'll want. Commuting takes more insulation than longer, faster rides. For most purposes, thin is fine (like a skull cap). Thick (like a wooly knit cap with your favorite sports team logo) is almost always overkill.
A long sleeve base layer that zips up the neck is ideal to keep the chill out. Keeping your core warm is key and the mid-weight top is the solution. For midweight, think of a thinnish sweater. Nothing really bulky. Maybe not even sweatshirt fabric thick. When paired with a windproof shell and a little aerobic activity, you don't really need that much insulation. There is a reason that the windproof shell is Tip #1!
For the legs, long pants are good when it's really cold (towards 40 degrees), but something that can give your legs a little breathing room is better when it's warmer (towards 50 degrees). Whether it's something cropped (like capri length) or full length but stretchy enough that you can push up to your knees if you really get warm. Cycling-specific leg warmers are fancy but far from the only thing that gets the job done. Just make sure that anything long is narrow fit at the ankle or that you have something to secure any loose fabric to keep it out of the chain.
Bottom line: Don't let Old Man Winter keep you inside sipping hot cocoa. Use these tips to keep your legs spinning through the cold and into spring.