Fleece is a synthetic material that feels lightweight, breathes well, and feels warm even if it gets wet. This combination of traits makes a fleece jacket ideal as either a mid-layer or a primary layer for a wide range of activities.
Fleece jackets come in a variety of numerical and relative weights. The higher the fleece weight, the warmer the fleece will feel. Ideally, you want a fleece that feels just warm enough for standing around but not so warm that it’ll leave you sweating when you’re skiing, snowboarding, climbing, or hiking.
For casual wear, almost any fleece will work as long as it feels comfortable and fits your style. For technical activities, look for a slim or athletic fit so you can wear it as a mid-layer. This will ensure that your fleece doesn’t feel too baggy beneath a shell jacket.
A fleece with a windproof membrane is more versatile and allows you to wear it as your primary (outer) layer in cold, windy weather. However, this will also compromise breathability for highly aerobic activities.