Knitting is one of several ways to turn thread or yarn into cloth. A series of loops are formed on a pointed stick called a knitting needle. Another needle is used to pull yarn through the loops to produce another row of loops, and so on. Work can proceed in the round or by going back and forth in rows. Knitting can also be done by machines.
There are two basic stitches: knit and purl. They are identical, except that a knit stitch is formed by inserting the needle in the front of the loop, while a purl stitch is formed by inserting the needle in the back of the loops. If only knits or purls are used when working back and forth in rows, the result is called garter stitch. Alternating rows of knits and purls result in stockinette or jersey stitch, the stitch most often used in commercial garments such as T-shirts. Different combinations of stitches can be used to form ribbing, cables, or other textures.
Someone who knows more about knitting should add to this. I know far more theory than practice.