Overview & Obtaining
Jackets are a middle ground between a duster and a parka, best suited to moderately cold biomes (not extreme enough to require 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 117 units of work.
Conclusion & Comparison
Overall, jackets are a middle-ground between dusters and parkas; requiring fewer resources and less work to craft, but offering less value in either extreme cold or extreme heat. In warm biomes, dusters are almost always more useful, since dusters cover more body parts and do a better job protecting from heat. In extremely cold biomes, parkas are often required to deal with ultra-low temperatures.
In moderately cold biomes though, jackets can be preferable to parkas in some cases Jackets offer better armor than parkas, so if you can get good enough insulation from the cold using jackets you're usually better off doing so.
Jackets offer a decent amount of insulation from heat (30%), and a great amount of insulation from cold (80%). The exact insulation in-game depends on the textile used and the quality of the crafted item. Assuming normal quality, a jacket offers: