The breathability of a jacket tells you how much water vapor, or sweat, can exit the jacket. The more a jacket can breathe the better it will be at keeping you a consistent temperature. Since the jacket is only letting out sweat, more breathable jackets are not cold, just more comfortable in all conditions.
Jackets with Very High Breathability (>20,001g) will keep you dry and comfortable in any conditions. These jackets breathe so well, they will keep you dry even under a full day of heavy activity.
Jackets with High Breathability (15,001-20,000g) are good for skiers and snowboarders who will work up a sweat from time to time. These jackets provide great breathability which keeps you dry throughout the day.
Jackets with Moderate Breathability (10,001-15,000g) are the most common, since they provide a good balance between function and the cost of technical fabrics. The jackets in the moderate category can keep you dry and comfortable during a full day of moderate activity and will easily handle sustained periods of high activity.
Jackets with Mild Breathability (5,001-10,000g) offer an adequate amount of breathability to remain comfortable for a full day of low to moderate activity and short periods of high activity. However, without careful layering you will become sweaty with high activity.
Jackets with Low Breathability (<5,000g) offer some breathability but will keep sweat against your body under moderate to high activity, causing you to become cold when you stop moving.
Jackets which are in the cateogry of Not Specified are made of materials that have not undergone breathability testing. This is common among casual and fleece jackets.
Jackets that are Not Breathable are specified by the manufacturer that they are not breathable, meaning that no water vapor will escape from the garment.