Here are a few comments in response to Practical Crocheter’s post below.
One way of understanding the fundamental complexity of crochet, if you are a knitter, is to think of each stitch being bound off before moving to the next. Such a view explains a lot about the characteristics of crochet. Anyone who knits knows that the bind off edge is inflexible, so if binding off is part of every stitch, it’s not surprising that the resulting fabric will lack the elasticity found in knitted fabrics.
Most knitters will also know that picking up a stitch creates a seam behind the new stitch. Every crochet stitch is “picked up” in the row below. While crocheted stitches do enclose the naturally resulting seam, they do not eliminate them, explaining the thickness of many crocheted fabrics.
Here’s an example. One of the simpler crochet stitches is called single crochet (sc). It is familiar to knitters as a nice option for edging knitted fabrics. If one were to accomplish an sc with knitting needles, the instructions might be as follows:
With one loop on your right hand needle, pick up one stitch in a swatch, turn. In the next row, knit two together.
In knitese: Row 1: CO1, pu1. Row 2: k2tog
Now, imagine following that instruction all the way across a row. With knitting needles it would be pretty awkward. Each of the basic crochet stitches is a variation on that concept, but with varying numbers of “rows” and “knit two togethers.”
In order for a knitted fabric to mimic the texture of a crocheted fabric, it must mimic (to varying degrees) the construction. Pattern stitches in knitting that look like they are crocheted often involve complicated increases and decreases in bizarre combinations because they are going through the process of “crocheting” with knitting needles.
Space. In knitting, all of the stitches are interconnected. If one stitch is dropped or the thread of one stitch cut, the stability of the entire fabric is jeopardized. In some knitting techniques, this fact is even exploited. Because every crochet stitch is bound off by itself, and is therefore its own fabric, there is space between the stitches. That space determines the drape and elasticity of the fabric--not to mention its potential uses--just as much as the stitches themselves. It is why crocheted lace looks crisp, even like cut work, and knitted lace looks mesh-like, almost like cobwebs.