What’s the difference?
Felting is the process of turning fluffy wool into a strong, thick fabric that will neither unravel (as knitting will) nor ravel (as weaving will).
You can begin by making the wool into yarn, or having that bit done for you, and using knitting or crochet to produce a shape which you then felt, or full, using heat, agitation, and soap to turn it into felt. This is how the traditional French beret is made.
The book Caveman Chemistry explains how the formation of raw wool into thread mirrors the formation of polymers. It also outlines the possible dangers of spinning and how to avoid them. This bit is just because all the chapters have a section on dangers (most of them being about making gunpowder, etc.) and the author didn’t want this chapter to be left out. You can tell he doesn’t have his heart in it.
This page clearly explains spinning, and shows you how.
I knitted two Sophie bags with the same yarn. The green one used smaller needles, and looked like normal knitting before I felted it. The gray one used needles which would be too big for this yarn under normal circumstances, and so it had to be felted down more severely.
We have put books in it to give it the rectangular shape we want. There are still no handles. You might notice that in the unfelted picture, the bag takes up the full width of the bench. After felting, there are a couple of inches of bench left over. So the bag is a good deal smaller, and much thicker, than before it was felted. It also can — until it dries — be manhandled into roughly the shape you want.
The green Sophie bag, with handles, is slouchier. It has not been felted as severely as the gray one. It is softer, and maintains its original knitted shape pretty much. Much of this kind of difference is controlled during the felting process — how much hot water and agitation do you use? The gray bag went through a full wash cycle in hot water and spent a few minutes in the dryer, too. The green one just had a bit of a rinse and spin.
To the right, you can see the felted fabrics. The stitches are gone, essentially. Both are felt now. Sigh. There is always something a little sad about that for me, although I have now gotten used to it. The exact texture you end up with in your felted stuff depends largely on the yarn you use. Things like cables and other texturework lose their character during felting.
A third example is the coaster below. It used to be a normal sweater, and has been so severely felted that it is not just felted — it is felt. I cut a circle out of it, something which of course cannot normally be done with knitting. Such thorough felting gets rid of the stitch texture entirely and blurs the colorwork, too. It no longer appears knitted. I felted a batch of old wool sweaters by completely washing and drying them in hot water, by machine. I cut them up and made a patchwork throw out of them, and coasters from the scraps. There is, by the way, an old CD in the middle of the coaster, to make it firmer.
Now, the hat on the right was never knitted. The fibers were pushed together so aggressively that they became felt without any intermediate steps. The shape of this kind of felting depends on using a form of some kind. It is then fulled — that is, worked and shrunken down. You could say that the knitted pieces were fulled, too. The swirl designs were put onto the surface of this before it was felted, and they were just pushed right into the surface by the felting process — which involved dribbling the thing around like a basketball. We’re talking serious agitation here.
Note that this process requires wool. You can’t do this with cotton, flax, fur, or even the fiber of the lovely acrylic plant. They will just laugh at you. That wool does this at all is sort of like magic. Or at least like chemistry.