Synthetic bags use synthetic (usually polyester) fibers as insulation in order to trap air and keep you warm. Synthetic insulation is water-resistant, quick-drying, easy to care for, and completely hypoallergenic. It also offers the greatest number of options for consumers, as synthetic fills vary greatly in bulk, weight, warmth, and price. The sleeping bags on backcountry.com display a temperature rating in the title. This is the EN, or European Norm, rating which is a standard used throughout the industry. This number is used to give you an idea of the lowest temperature at which the bag will keep you warm.
While not as lightweight or packable as down, synthetic insulation is water-resistant and still insulates when wet, making it ideal for wet environments. Plus, it's considerably less expensive than down.
Mummy-shaped bags taper towards your feet to reduce volume and hold warmth closer to your body. These bags can feel restrictive to some. A semi-rectangular bag can offer more room to move.
If you're facing wet conditions, look for a bag with water-resistant properties. DWR coatings on the outer fabric repel light moisture to protect against mists, drizzles, and snow.