First thing's first - what exactly are knit fabrics and how are they different from wovens?
(close up and enhanced pic of a woven quilting cotton)
Woven fabrics consist of alternating yarns inter-weaved perpendicular to each other. Remember doing basket weaving in elementary school - same concept, only smaller and with tiny yarns on a big giant loom.
(Left: close up of the right side of a single knit jersey, Right: the wrong side of the same jersey)
Knits are usually one yarn (sometimes more than one) that is knit together in loops. Knitting machines work using the same method that you would use to hand knit and purl. A basic plain knit has vertical ribs on the right side and horizontal lines on the wrong side. The inherit nature of the yarn being knit together makes the fabric stretchy. Knits generally don't fray like wovens but sometimes the edges curl.
(Left: Jersey flat, Right: Jersey stretched widthwise)
Some woven fabrics can be slightly stretchy but that is due to special fibers like spandex or lycra (think about your "stretch" jeans") but knits on the other hand are stretchy due to the process of making the fabric.
***This is the shortest post of the series but it is important to get the definitions out of the way. Tomorrow we'll talk fiber content, types of knits and picking patterns appropriate for knits!