Knitting yarns very in texture, thickness, and fiber type. The look and feel of the hand-knitted clothes depends on yarn more than on the skill of the knitter.
My grandfather had a vest made out of the homespun dog wool. The dog didn't mind, even seemed flattered. I had a goat sweater hand-knitted for me by old women from the small Dagestan village in two days while our hiking group camped across the river. Camel wool is sold in unspun bundles along the great trans-Eurasian railroads -- in Asian parts, of course. Locals insist it is very healthy, especially for your feet. I think they are right. Camel socks feel warm immediately, almost magically. I knitted my first sweater at thirteen to prove to my grandmother I can persist and finish what I started. It was mid-thigh long, dirty purple nondescript wool sweater with cables. It took me two months to knit it and I never wore it.
Acrylic, nylon and polyester synthetic knitting yarns available in craft supply stores are cheaper than wool. They come in various colors and textures and are often considered suitable for beginners. They yield uniform, clean-looking relief. They typically are colorfast. However, I believe knitting or crocheting with cheap synthetic yarn is a waste of time: it is just as unnecessary and time-consuming as knitting from wool, and this yarns feel rubbery and slippery in my fingers. The knitted clothes will look exactly what they are: cheap synthetics that lose its looks after three washes. Best synthetic insulator is polyester fleece, and it looks stylish and funny with simplest applique and tassels. Textured novelty synthetic yarns are fun and often are quick to work with.
Wool knitting yarns are most common. The quality of the yarn depends mostly on the length of the fiber. Wool comes from sheep (mostly), goats, llamas, alpacas, camels, and rabbits. Best wools come from animals living in cold, harsh climates. Cashmere is a celebrated sheep wool from the inner Mongolia (China), fine, lightweight, lustrous and warm, and costs its weight in gold. Wool is often blended with synthetic fiber or silk, also different types of wool sometimes are blended together. Worsted Merino wool is warm, comfortably thick for knitting and gives beautiful relief, which makes it very gratifying hand-knitting yarn.
Thickness of the knitting yarn: usually, the thicker the yarn, the thicker knitting needles you will need. However, some knitters use thick needles to make loose, fluid fabric out of fine yarn. Since most models have specific loop/row numbers for parts of the clothes and for the patterns, it makes sense to adjust the needle size to get the required stitches comfortably. Knitting is very monotonous and benefits greatly from practice. My mother-in-law, a most amazing knitter artist, uses long steel 2mm (size 0 US, 14 UK and Canada) needles for any yarn. She is used to these needles and comfortably knits even thick bulky unspun yarn (aran) with them. Yarn thickness is expressed in numbers, 1 is finest "sock" or "baby" yarn, 6 is thickest "roving" or "roping". Knitting yarn packages have also numbers for recommended knitting needles sizes and expected stitch and row numbers for a 4" square.
Quantity of the knitting yarn: knitting yarn is sold by length and by weight. A warm sweater weighs between one and two pounds, a fine feminine cardigan about ten ounces, a baby blanket about a pound. Knitting patterns often list the required quantity.
Hand-knitted clothes usually require hand washing and flat drying. I wash my hand-knitted sweaters with the same shampoo I use myself: wool is hair, after all.
See also: knitting needles.