Sleeping bags are filled with a down or a synthetic fill.  Each type of filling has benefits and downsides.  Down fill is very lightweight, packs very small, and is warmer than synthetic (because it insulates well) until it gets wet.  Once down is wet it is not longer warm and takes a very long time to dry.  Synthetic fill is a heavier material and does not pack as small as down.  On the upside, synthetic fill stays warmer when it is wet and dries faster than down.  When deciding between a down sleeping bag versus a synthetic sleeping bag, consider your needs for space, weight, insulation when wet and drying time.