Acrylic, nylon, spandex, and polyester are all examples of fibers made from chemicals designed to closely resemble those sourced from either animals (such as fur, wool, or cashmere) or vegetables (such as cotton or hemp). While synthetics tend to be less expensive than their natural counterparts, they're non-renewable and typically not as warm or long-lasting as other fibers.
Woven together from fibers of cotton plants, this lightweight material is the most common sweater fabric and is best used for layering. It's not the best option for chilly temperatures, but it is one of the easiest fabrics to maintain and clean (most cotton sweaters can be thrown into the wash without a second thought).
Derived from the coats of various animals, the term "wool" covers a surprisingly broad range of textiles, but the most commonly used form is from sheep. Even among sheep's wool, there are several categories, such as merino, Shetland, and lamb's wool, with merino being the softest (merino sheep are one of the world's most ancient breeds!). Wool can be a bit on the pricier side, but not only is it one of the warmest fabrics, it can also can hold up to 30 percent of its weight in water without feeling wet.
Cashmere is technically a type of wool, but the fibers are specifically extracted from cashmere goats. If you've ever felt cashmere, you'll understand why it's considered one of the most luxurious cold-weather fabrics: it's exceptionally soft and warm. Unfortunately, the delicate fabric is also one of the most expensive, as it takes fleece from several goats to create just one sweater. But if you're looking to invest in a new sweater this fall, a fuzzy cashmere number won't let you down.
Blends are combinations of any of the above. Clothing manufacturers will often use a blend of several fibers to get the best of both (or many) worlds. For example, wool and nylon are often mixed to give the soft wool fabric more elasticity. Or materials may be combined to lower the price and make them more durable, as in a cashmere and nylon blend.
Softness: N/A (Depends on the blend.)
Warmth: N/A (Depends on the blend.)