Woolen vs Worsted
....The main difference is whether you let the twist enter into the drafting triangle or not. Whether you do a short draw or a long draw, if the twist enters the drafting zone, it is woolen technique. If you keep the twist out of the zone, it is worsted technique. Everything else is details.
That being said, here are some more details. Woolen mostly uses shorter fibers that are usually carded, and lay less parallel than combing. For "true woolen", it uses carded rolags. And it will be a lot loftier and lighter if those are spun "long-draw", which uses the twist itself to draft out the fibers, with the working area being much longer than the length of the fibers.
Worsted is usually spun with fibers that run more parallel, which is usually achieved by combing. Combing is often used with longer fibers. "True worsted" uses combed top or combed locks, which removes all the trash, and shorter and weaker fibers. It is done with a short stroking draw that keeps the twist out of the fiber supply. The working area is limited by being a little longer than the length of the fiber. The desired result is stronger and less poofy.
Limits are placed on the end result by both the preparation and the technique. Carding produces a poofier yarn and combing produces a smoother one. Woolen technique produces a poofier yarn and worsted produces a smoother yarn. Poofier yarns have a lower fiber to air ratio, and are lighter. Smoother yarns have a higher fiber to air ration, and are heavier.
Then there are a lot of other techniques that produce all manner of thread and yarn that are somewhere in-between. There is flicking and spinning from the fold, carded clouds, batts, etc.
And that's just the info as I know it, put as best I could into words.
Now, when I decide whether to comb or card, it will usually be decided by the staple-length, and my desired result. I will usually card shorter fibers and spin them woolen style. I will usually comb longer fibers, but then decide whether to spin woolen or worsted. I do a lot of designer blends on my drum carder, some mix very short fibers with fairly longish fibers, and I like to spin them woolen and long draw.
As artists, we have a lot of different options and tools in our box. Then there are the colors! and the sparklies! and the noil!