When it comes to spinning, there are only two factors the spinner must control to produce the type of yarn he or she desires. They are: size (diameter, grist) and texture.

Grist is the number of units of length there are to one unit of weight. With yarn the unit of length will be either yards of meters; the unit of weight could be ounces, pounds, grams or kilos.
In the US, for yarns the most common unit of length would be yards and the unit of weight would be a pound. So grist equals the number of yards per pound or meters per kilo. For yarn spun at 50 yards per ounce, multiplying 50 yards (the length) times 16 ounces (the standard weight unit) would give a grist of 800 yards per pound. Controlling grist in spinning is discussed here:
(Controlling Yarn Size)

Controlling the texture of the yarn starts with the preparation of the fibers for spinning. Good clean fleece that is well prepared is essential to producing good yarn. Processing faults can create unwanted texture in yarns.

Some of the things that produce unwanted texture in yarns are:

1. Vegetable matter in the fleece. Foreign matter in fibers is due to improper skirting and picking. The little bits of hay, grass, seeds, etc. left in the fibers will be drawn into the drafting zone when spinning and will form little lumps as they become trapped in the yarn structure.

2. Second cuts. Second cuts are the short bits of fiber that come from the shearer going over an area of the sheep’s body with the shears for a second time. These short bits of fibers should be removed from the fleece at skirting as they will form neps in the carded fiber.

3. Washing. Poorly washed fleece may be clumped, partially felted, tangled, and may not draft well. It may also be further damaged by carding which may cause neps.

4. Poor carding. Sloppy carding makes uneven batts with the fibers going every which way rather than being nicely organized. A poorly carded batt will not draft well or evenly.

5. Blending. Blending differing lengths of fibers can cause texture. Longer fibers may tangle and form neps while shorter fibers may clump together creating noils.

These things are not the type of things we want in our yarns, especially not if we want a smooth yarn. But sometimes we want a textured yarn. What then?

While items 1 through 4 above are obviously not going to create a nice textured yarn because they are due to faulty craftsmanship, number 5 raises some very interesting possibilities for texturing our yarns.

Silk noils, which are short clusters of silk fibers, blended into wool adds a lovely nubby texture to the yarn. Garnetted yarns (tweed, knickerbocker) are also created by blending short fibers into longer ones, purposely creating neps to add texture to the yarn.