Knitting is the act of using needles to manipulate yarn into loops and then stitches, creating a piece of fabric. An individual loop all by itself quickly unravels if tugged. But if that loop is pulled through another loop and itself has a loop pulled through it, that loop can no longer unravel. Doing this repeatedly produces a piece of fabric.
When done by hand, knitting requires two stick-like needles, yarn, and hand-manipulation. After casting on a row of loops on one needle (the holding needle), the other needle (the working needle) is used to turn those loops into stitches and then store the newly created loops. Once the working needle has knit through all of the loops on the holding needle, the two needles switch roles, such that the holder becomes the worker and vice versa. This process repeats until the piece is finished and it's time to bind off. Stitch formation occurs by inserting the working needle into the loops created in the previous row on the holding needle, then wrapping the working tail of yarn around the working needle, and finally maneuvering the working needle and the wrapped yarn through the loop on the holding needle and pushing it off, forming a stitch underneath the new loop.
Knitting can also be achieved using a machine, which is similar to hand-knitting in the topography of the fabric produced, but very different in execution. A flat-bed knitting machine has a needle bed that contains a row of individual hooked needles that are each responsible for holding loops and forming stitches in the corresponding wale (or column). In this way, knit stitch formation occurs by extending the working needle far enough that the loop it is holding slides down off of the hook, opens the latch, and rests at the base of the needle. The extended needle grabs onto the working strand of yarn with its hook and retracts such that it passes back through the old loop, which closes the latch as it slides over and off of the hook. To form a full row, the working tail of yarn is carried across the bed and laid down over each active loop as the carriage selects the correct needle and triggers the stitch formation process. This mechanized method of fabric production is exceptionally efficient and creates room for exciting possibilities with computer-aided fabrication techniques.
Not all knitting machines are completely automated, though, as is the case with domestic knitting machines, which require manual carriage movement and some hand manipulation (but may also have electronic components). That being said, knitwork and the documented resources are designed for computerized flat-bed knitting machines, which are fully automated and programmable. Computerized knitting machines typically contain a number of additional parts that advance their capabilities, such as carriers, rollers, sinkers, sliders, inserting and holding hooks, and grippers (though features vary across different machines).