Circular knitting needles are two knitting needles that are connected with a flexible cable. The needles were often made in blacksmiths, where steel wire was cut to length, hardened and then smoothed at the tips to allow knitting from both ends. The remaining needle is used to knit the loops from the first needle, and when that needle is empty, knit with it from the second needle and so on. The length is measured from needle tip to needle tip, and generally you want to use needles that are slightly shorter than the circumference you want to knit.
You can find smaller cables, but most knitters use the magic loop or switch to DPNs for small projects like socks.