Circular knitting (also called knitting in the round) is a knitting technique where a round, tubular fabric is created. As one knits in rounds / spirals, the consequence for the fabric is to be seamless. With ciruclar knitting, the fabric is always knitted from the same side.
Circular knitting can be done by hand or by machine. When done a machine, one needs circular knitting machines. The creels of circular knitting machines are usually arranged in a circle, just like the thread guides and the needles.
Per cone that is put on the creel, there is one thread guide and one needle. In case the yarn breaks or there is a knot in the yarn, sensors will detect that and stop the knitting process.
Typical circular knitted products are socks or mittens. Pantyhoses are knitted on a circular knitting machine as well, but are rather allocated in hosiery.