Sleeping bags come in three basic shapes: rectangular, the "mummy" or cocoon style, and tapered. The rectangular shape is self explanatory and offers a roomy design. It comes in a single or a double and the double is large enough for two average size people to sleep in. The cocoon is a form fitting style that is smaller and lightweight, but still offers the same temperature rating as comparable rectangular bags. The tapered style is wider in the shoulder area and narrows at the bottom.
While rectangular sleeping bags are probably the most popular and the most readily available style, experienced campers and adventure seekers favor the tapered and cocoon designs. These styles are easier to pack and carry. They are quite snug and they work well in intense conditions.
Sleeping bags also come in a range of materials depending on their use. A flannel-lined one provides warmth but also provides softness next to the skin. Nylon bags "breathe" better and do not retain moisture, so they will dry more quickly than other styles. Poly-cotton blends are probably the most durable option and work best in extreme terrain.
There are also differences in the way sleeping bags are sewn and filled. Down fill provides great natural warmth, but cotton, polyester, and new types of synthetic fills are also available. Many new synthetics use a "hollow fiber" method that keeps the bags light and allows for good compression when packing, while still offering excellent warmth. Synthetics are affordable and easier to care for than down.
The way sleeping bags are sewn is also important to consider. Some are made like quilts and the entire bag is stitched into small pockets of fill. This keeps the fill in place, instead of letting it all fall into one section of the bag. Other styles sew "tubes" into the design, using long narrow pockets instead of small squares. Tubular designs are also intended to keep fill in place.
You can also find great sleeping bags for children. These designs are smaller, child sized versions of regular sleeping bags. Most are made in the rectangular style, and you can find a wide variety of designs and trademark characters. Some are made to coordinate with bedroom accessories and are perfect for use as extra sleep space when your child has a sleepover.