The type of insulation you choose will depend on the amount of warmth you want the jacket to provide and the weather you are going to be using it in.
Down insulation is natural insulation that offers amazing warmth to weight ratio. While high fill down jackets may feel bulky, they are the warmest jackets out there. Lighter fill down jackets make a fantastic layering or warmer weather jacket.
Fleece insulation is a fantastic combination of lightweight and warm. Fleece insulation will either be bonded to the inside of a jacket or used to make up the whole jacket.
None simply means the jacket offers no insulation. These “shell” jackets are typically designed to be incredibly water and windproof allowing you to layer under the jacket so you always have the right amount of insulation for the conditions.
Synthetic insulation is the most common form of insulation and is often called by its brand name like Thinsulate or Primaloft. Regardless of the brand, it is a synthetic layer that provides warmth to your jacket and still performs well when wet or damp. So if you get caught in a storm, you’ll still be warm.