I am a spinner and can make my own yarn. Making yarn is quite a technical process. I spin singles to start, they have a life of their own and go every which way, can curl back on themselves. As a yarn, singles are not at all strong, would easily bobble and break. The singles are kept in order on a bobbin. I never knit with singles. Arty people might and a knitted piece would tend to twist sideways
Two bobbins can be spun together, this is a simple 2 ply. The spinning is in reverse ie anti-clockwise. It settles the singles and makes a more compliant yarn but still not incredibly strong
Three bobbins make a 3 ply and this gives rise to a stronger rounded yarn. I mostly make 3 ply. There are basically 2 types of ply, woollen and worsted, woollen is airy and light,not so many twists, excellent for cuddly jumpers, shawls, and hats Worsted is tight and more twisted and is hard wearing, very suitable for the likes of socks
Pure wool has many good properties. It only needs airing rather than washing, the air in the fibres is extra insulating. Once used, it grows again on our sheep.
There is a knitting thread in the arts and crafts forum, main thing is always but always to make a tension square, then wash and only then count the stitches and rows