I am often explaining the difference between knit and woven fabrics to my clients, so I thought I'd put a simplified version of all that are textiles in writing for reference.
All fabrics are made from fibers. Fibers can be natural (silk, linen, cotton, hemp, wool, alpaca, cashmere, angora, etc) or synthetic (rayon, nylon, polyester, spandex). Some fibers are synthesized from plants, and so despite being synthetic, they are sometimes called "natural man-made fibers" (bamboo rayon, tencel, modal, cupro).
Spandex fibers are the only fibers that currently provide the most reliable stretch in fabrics. When spandex is used, it is generally the core of the yarn strand, and the other fibers are then spun around the spandex fiber.
All fabrics are made from yarn. The yarns can be composed of a variety of fibers. When fabrics are made from a blend of fibers (say Poly/Cotton/Spandex, for instance) most of the time the yarn is spun with these fibers mixed together on the same strand.
Yarns can be thick or very thin. The width of the yarn strand is known as a gauge.
Almost any gauge of yarn can be knit or woven into a textile. Some heavy gauge yarns, however, can only be sweater knit.
Yarn can be knit or woven as PFD (prepared for dyeing), which is a natural color, or the yarn can be dyed ahead of time to create a desired color. When stripes are desired, two different solid colored yarns are used on the same fabric. When heathers are desired, the yarn fibers are either dyed a variety of colors before being spun together, or the entire yarn strand is dyed one color, but because the fibers react to the dye differently, the yarn itself gets a heather look. When space dyed looks are desired, a single strand of yarn is dyed a several colors every few centimeters or inches.
Knit fabrics are made with a single strand of yarn that is looped to itself. Think: ball of yarn and two knitting needles.
Knits generally have what we call 'mechanical stretch,' because since the yarn is looped to itself, when pulled on, the fabric will spring back to its original condition, even without spandex fibers present in the yarn. Over time, most fabrics with 100% natural fibers do not keep their shape as well because of the nature of the fiber, but anything knit that is blended with synthetics has a higher chance of keeping its shape longer. When spandex is used in knit fabrics, even with natural fibers, the fabric is made to stretch and will maintain its shape fairly well.
There are many types of looping techniques that can be used in knitting. The texture of the fabric is dependent on the loop technique used. The weight of the fabric is dependent on the gauge of yarn used.
Knits tend to be more fragile than woven fabrics.
Woven fabrics are made using two yarns. One going horizontally across the width of the fabric, and the other going vertically across the length of the fabric. The yarn across the width of the fabric is called the weft yarn, and the yarn running the length of the fabric is called the warp yarn.
Woven fabrics can also have some element of mechanical stretch, but this stretch is generally much less than that of a non-stretch knit fabric. When spandex is used in woven fabrics, it can either be used only in the warp yarn or only in the weft yarn, or in both yarns. When spandex is present in only one of the yarns, it provides what we call a two-way stretch. When spandex is present in both of the yarns, it offers four-way stretch.
Weft yarns are used to weave in and out of the warp yarns. This is done by using a loom. The warp yarns are set up on the loom, and the weft yarn is then woven in and out of the warp before it is tightly pushed together by a reed.
Though mechanical weaving machines are ubiquitous in this day, many woven fabrics are still done on hand looms like the image below.
Because woven fabrics are made from two yarns, two colors of yarn can be used to make patterns such as plaids, herringbone or tweed looks. When denim is made, it is often made with white or natural weft yarn, and blue warp yarn, which is why traditional denim has one color on the face of the fabric, and another on the back of the fabric.
Also as are result of the warp and weft yarns, woven fabrics can be very, very strong, even when two thin yarns are woven together. Silk habotai for example, is very thin, but because silk yarn itself is rather durable, and the thin yarns can be woven very tightly together, the fabric is very tear and snag resistant despite its lightness in weight.
Here are some cool illustrations I found from Carrie Parry that show how some natural fabrics are woven from fiber to finished product
Illustrations from Carrie Parry
COURTNEY CADY, SHALLOW FASHION 2016