You will find that most athletic and outdoors clothing is made with synthetic materials for their improved properties over traditional natural fibers such as cotton.
There are may types of synthetic and artificial fibers including rayon, spandex, lyocell, modal, nylon, and polyester. Most of what you will find in the clothing we talk about is nylon and polyester (and some of the more unusual, naturally derived artificial fibers). Nylon and polyester are made from petrochemicals, while the artificial fibers come from cellulose derived from plants. These fibers are formed by extruding liquid polymers into air or water.
A lot of the properties of synthetic fabrics not only come from the fiber itself, but also the weave/knit of the fabric (think fleece vs. a wicking workout shirt, both are made from polyester). In many cases, synthetics can also be blended with natural fibers (like nylon core merino wool).
Nylon vs. Polyester
Nylon is a stronger fiber than polyester, making it a great candidate for bags and blending with other fibers for strength.
Polyester has the upper hand in moisture control. While both fibers are hydrophobic (they repel water), nylon absorbs more water than polyester, resulting in a fabric that can be heavier when wet and take longer to dry.
The naturally derived artificial fibers, such as lyocell and modal, (remember these are plant cellulose, but are still manmade) have many of the same properties as natural cellulose fibers (cotton), but have some advantages such as softness or wrinkle resistance.
There are many benefits synthetic fibers can impart on clothing including durability, stretch, wrinkle resistance, moisture wicking, water resistance, wind resistance, and stain resistance. These properties make the fabric popular for harder use garments (like outdoors clothing and activewear). Over time, these fabrics have started to make it into dress and casual clothing for the same reasons. They can even make your blazer more comfortable, durable, and have the ability to go in the wash.
Synthetic fabrics tend to smell more quickly and retain smells longer than some natural fibers (in fact, it has been shown that some bacteria that produce odors love to live on polyester).
Synthetics also can be more staticky than natural fibers, and they also melt when exposed to heat/flame (or an iron).
Synthetics certainly can mimic some of the performance characteristics of our favorite natural fiber, merino, but also can impart some downsides. Mainly, synthetics are used when durability or cost are top considerations.
Where We Like It
We like synthetic fabrics for their stretch and durability, especially in garments like pants, shorts, and outerwear. They can also be great blended with fibers such as merino, with the resulting fabric having unique properties such as stretch or more durability. One great example is NYCO, a nylon/cotton blend. Some others include merino with a nylon core and merino blended with lyocell. These fabrics all retain some of the good properties of their natural fiber component while improving performance with the synthetic fiber.