The link above seems to be more weaving/spinning oriented. For instance, the definition of "worsted" is entirely different for knitting yarns. Well, a knitting yarn *can* be worsted by that definition - long-staple, combed, tightly spun - or "woolen", and still either "worsted" in knitting weight or some other weight entirely.
In knitting terms, "worsted" is a weight of yarn. That weight is 220 yards per 100 grams, and knits up to approx. 5 st per inch. Other terms you might see referring to weights are: fingering, sock-weight, bulky, and a few others. I think knitting.about.com has a chart describing the technical definitions of each.
As for ply, to be honest, I'm not sure why knitters use this relative to weight. You can get any weight yarn (worsted, fingering, whatever) in any number of plies. I have several yarns in my basket, all worsted weight, some one ply, others four ply, others three ply. Now, these will give you a slightly different look - try knitting a swatch in a single ply yarn and another in a four ply yarn and you'll see the difference. But, they're still the same weight and will give you the same result in terms of gauge and ability to felt and so forth. In weaving yarns, ply is part of the weight definition. You get two numbers, such as 20/2. (I believe that those numbers are reversed outside the US. We always have to do things differently...) The first number is the weight (yarn count, taken from the number of yards per pound) of a single ply, the second number is the number of plys. 20/2 would be the same thickness as a 10/1 yarn, but would look different - it would have two plies each half as thick as the 10/1. This is an infinately more logical system for catagorizing yarns, and I am frustrated trying to identify yarn weights in knitting. My guess is that other countries (correct me if I'm wrong) use the same identification system for knitting yarns as for weaving yarns, and that's why some folks find "ply" to be an important number.
<musing>My only question regarding ply is whether plied yarns cause the final fabric to distort less since much of the tension from the twist is released by plying. Think of when you sit, bored, in class idly twisting up a lock of hair. If you let go, the tension in the twist causes it to untwist. But, if you let it twist back up on itself, it becomes a two-ply twist of hair, those plies have twisted the opposite way from the way the hair was originally twisted, (it's called S or Z twist because of the way the fibers go, i.e. you look at the yarn and if they go the way of the middle bar in an "S" it is S twisted, and similarly for "Z". If your single plies are S twisted, you will get a Z twisted yarn.) Now, the tension on a yarn is much less than that on your hair because the twist has been "set" through a finishing technique such as steaming. But, there is still some tension. So, since there is less one-directional tension on plied yarns, does this make such yarns better for things like Fair Isle sweaters that are knit in the round and tend to twist on the body?</musing>
