Overview & Obtaining
Jackets are less insulating, but also lighter than the likes of Parkas and Dusters in the respect that they impose less of a movement speed penalty, and don't hamper global work speed unlike parkas. They can be purchased (as with any other piece of apparel), crafted, or stripped from Outlanders and Pirates. Starting colonists may also spawn wearing a synthread jacket.
As a complicated garment, Jackets can only be made at tailoring benches, which requires complex clothing to be researched in order to be constructed. A jacket requires 70 of any textile, and 200 seconds of work (12,000 ticks) to be created, assuming baseline global work speed, consciousness, sight, and manipulation.
Conclusion & Comparison
Overall, Jackets are essentially outer-layer garments on a budget; requiring fewer resources and less time than dusters or parkas, but are overall less protective and insulating alongside. Therefore, it's more practical to have Dusters as they cover more body parts with a slight deficit in sharp protection, with the addition of thermal insulation. Parkas are somewhat situational as their optimal application is in harsher cold climates, whereas woollen jackets will suffice in milder climates - and (as previously mentioned:) jackets don't impose the same movement or global work speed penalties that parkas do.
The following table(s) will display how good a normal quality Jacket is at insulating when made from various materials...