The nitty gritty of yarn classifications!
"woolen" versus "worsted" spun yarns. The difference is not particularly relevant to knitters, but it becomes a very big deal if you (like me) begin learning how to spin your own yarn.
To make things more confusing, this is not "woolen" as in "made of fibers from a sheep." Nor is it "worsted" as in "worsted weight yarn." In the context of spinning, there are secondary definitions of both woolen and worsted - definitions that you are not likely to have encountered before you start learning to spin.
The difference between the two is the alignment of the fibers, before they get drafted and spun up. With a worsted yarn, the fibers were combed into tidy parallel lines before you start spinning them. With a woolen yarn, the fibers were one big jumble before you tugged them out in the drafting process.
Typically, a worsted yarn is made with a long-staple fiber. It's easier to get the fibers to lay flat and parallel when they are longer. A woolen yarn is usually a shorter-staple fiber, and the classic way of preparation is to hand-card it (or use a drum carder) and then roll it up into those fiber burritos called rolags.
When you spin from a rolag, you are pulling the fibers at a 90 degree angle to the grain. Whereas when you spin with a worsted fiber, you are pulling the fibers off lengthwise.
Now, one problem here with my nomenclature is that, even though I am using a nice tidy rope of fiber prepared by an expert (Potluck Roving by Ferndale Fibers), it still gets really wadded up in my hand. I'll start out with a nice little rectangle of orderly fiber, but after a few minutes, it's a big sweaty blob of stuff in my hand, and darned if I can figure out which end is which.
As a general rule, worsted yarn is tidier and smoother than woolen yarn. You would expect this, since it starts out smoother, as well. However, woolen spun yarn has the advantage of being fluffier. Because the fibers are more jumbled, they create air pockets that are better for insulation, and they are loftier and softer. Woolen spun yarn is also easier to felt, since the fibers are easier to lock together when they are already jumbled up a bit.