A fleece blanket is an insulating blanket made of a synthetic fabric blend. The term "fleece" often causes confusion, because it is sometimes used to describe part of the wool of sheep. In this context, however, it refers to a specific polyester fabric with no wool content whatsoever. Polar fleece is the most well known type, though a number of varieties are available.
The fabric of this type of blanket is made by taking polyester yarn and twisting it tightly. The resulting material is then brushed with rough wire to tighten it and raise the surface a bit, then sheared to even the entire thing out. Virtually all types of fleece blanket are made with a high grade of fabric known as non-pill, which doesn't ball up after being washed a few times. In general, the surface texture of this material is nicer than other types of fleece.
Good blankets can come in a wide range of textures, each processed slightly differently. The top side may be almost smooth or may have a nice velvet feel to it, while the bottom side is usually even smoother, with a texture greatly resembling felt. Many people make their own blankets because of the ease of using fleece as a "no-sew" material. Beginners often like this fabric because it never unravels the way most others do, making it difficult to ruin.
There are three main weight categories of fleece that can be used in a blanket, depending on how warm the maker wants the final product to be. Microfleeces are the lightest and are not really suitable for anything but a light blanket meant to be used underneath a heavier one. Hundred-weight fleeces are the next weight category, and are about as heavy as an average pair of sweatpants. Medium-weight fleeces are often used for jackets and vests, and are ideal for a blanket used in cold conditions.
Fleece is a great material for blankets because of its excellent weight-to-insulation ratio. It is soft and comfortable, retains heat well, and is light enough to be carried on backpacking trips or stored away easily in a closet. In camping situations, the polyester helps to pull moisture away and keeps the campers drier than other blanket materials. While perhaps not as ideal as some high-tech alternatives, fleece offers many benefits for a low price.