Most bags are rated according to the lowest temperatures in which they provide comfort, but there is no set standard in the industry. The rating is determined by the manufacturer. Goose down is a natural fiber that has long been held as the standard as lightweight, comfortable, and warm. They compact easily into small stuff bags, mold easily around a camper's body to retain heat, and are extremely durable. The down side is that they can be miserably uncomfortable if they get wet, take longer to dry, are more expensive than most synthetics, and are most safely cleaned professionally.
Synthetic fills provide enough warmth for most casual campers.
Follow the manufacturer's guidelines for temperature rating, but adjust it according to your personal sleeping habits. If you know you are a cold sleeper, get one rated at a lower degree than you will be camping in. Any sleeping bag can be made warmer by using a purchased or home-sewn flannel liner, and some bags come with their own. Sleeping bags made for backpackers tend to weigh 6 pounds or less, so if you think you might want to pack into a camp site, look for a lightweight sleeping back. If you perspire heavily, you may prefer a cotton exterior, or at least an absorbent interior fabric.