Overview & Obtaining
Parkas are large, thick jackets which protect from the bitter cold of boreal forests, tundras, ice sheets, or the ever-perilous sea ice. Parkas insulate from cold exceptionally well, offering 2.5 times as much as the next best insulating clothing (jackets).
Parkas can be purchased from traders, crafted, or stripped from Outlanders, Pirates, or Tribals - this being the only garment that can be found worn by all existing human factions. When starting a tribal faction, there's a chance one of your starting tribespeople will spawn with a cloth parka equipped.
As a complicated garment, Parkas can only be made at tailoring benches, which requires complex clothing to be researched in order to be constructed. A parka requires 80 of any textile, and 134 units of work.
Conclusion & Comparison
Parkas are the most insulating garment in the game, offering an absurdly high amount of insulation from cold relative to other clothing. This insulation comes at a cost though: less armor than both jackets and dusters (20% vs. 30%), and no less coverage of the body (dusters cover the legs).
Parkas are a necessity in cold biomes, especially when starting out. But the extra armor of a jacket or duster may be worth it if you have good enough textiles and a craftsman capable of making high enough quality jackets or dusters to insulate from the cold.
Parkas offer no insulation from heat, but an abusrd amount of insulation from cold (200%). The exact insulation in-game depends on the textile used and the quality of the crafted item. Assuming normal quality, a parka offers: