Down is used for insulation in sleeping bags and jackets.
Down is the lightest material relative to the amount of warmth it provides because it offers a greater amount of loft. Down comes from geese and ducks.
When wet, down provides very little insulation because it looses its loft (the feathers stick together). Down's main competitor, synthetic materials, provide more insulation when wet, but are heavier.
Down products are normally more expensive than synthetic products. The added cost is worth it though when every ounce counts.
Fill rating: Any down product should come with what's called a fill rating. For example, you might see a down sleeping bag listed as having "600 Fill (or some manufacturers use the word 'Power') Down." This is a measure of the down's loft. The number listed is the amount of space, in cubic centimeters, one gram of feathers will take up. For example, one gram of feathers of 600 fill down will fill a volume of 600 cubic centimeters.
Any man-made fiber can be considered a synthetic fill. Synthetic materials are used to make sleeping bags and jackets
(as well as much more). Synthetic insulation provides warmth even when wet (an advantage compared to down). Although synthetic materials are cheaper than down products, they are also heavier than down products.
has become almost a household name over the years. They developed a long hollow fiber that is hydrophobic (doesn't soak up water) and is able to insulate well. Because each of the individual fibers are hollow, they provide loft (and therefore insulation). There are several generations of polarguard fills that differ in the cavity size of the center of the fiber. The products listed below are in order of the smallest to the largest fiber cavity size (or from lowest to highest loft).
is a short hollow fiber that is used primarily in gloves
It has better thermal properties than polarguard's products, but because of the short length of the fiber it tends to migrate more. Since it moves around so much, its optimal use is in tight compatmentalized spaces, like gloves, boots,
etc., but not in sleeping bags.
is a collaboration between Polarguard and Mountain Hardware. It takes the exceptional thermal insulation to weight ratio of PrimaLoft and makes it into a long fiber version suitable for sleeping bags. In my opinion, it is the optimal synthetic fill.