Polyester fleece comes in a variety of numerical and relative weights. The higher the fleece weight the warmer the fleece will feel. A 100-weight microfleece pullover can serve as a thin mid-layer or even a baselayer, while a 300-weight fleece jacket makes an excellent mid-layer on really cold days or an outer layer on days that aren't too windy.
While fleece can be quite warm, it usually doesn't offer much protection against the wind. Some fleece jackets feature a windproof membrane that makes them more versatile and better for use as a primary (outer) layer in cold, windy weather. However, this will also compromise breathability for highly aerobic activities. Many fleece jackets now feature a smooth knit face on the outside that's a touch more weather-resistant than traditional fleece, but just as breathable.