a close up of a braided cord. nylon roving was directly braided into this structure.
A braid is a structure that is created by interlacing singles, plied yarns and/or filaments in a flat, but more often, tubular structure. The most noticeable difference between a plied structure and a braided one is that the braid requires materials to travel in two directions around a center point. A tubular braid is a hollow structure and can be left hollow for particular applications and has particularly good mechanical properties for artificial muscles. These are created because, with slippery materials, the braid will increase its cross-sectional diameter when compressed, and narrow when stretched (a tubular scissor linkage).
Yet, we often find braiding as a method of adding strength around a core, for instance, in the creation of climbing rope or as a decorative "shell" for a mundane appliance cord. The advantage to braiding is that it makes it slightly easier to combine several different materials (e.g. conductive and non-conductive materials) as you have a bit more control over tension within braiding than you do with spinning.
Braiding can be performed with in spun yarn, filament, or even more complex structures such as knitted cord, as I found when I was disassembling some old shoelaces:
A Deconstructed Shoe Lace: the red and green cords are the braided outer sheaths and the multi-colored strands were knitted insides of the lace.
Because of the multitude of factors that can be used in a braid, I do not know of any standard measurement across all applications. Measurements of braided rope or cord express the measurements of the various components involved. For instance, I found this set of measurements for parachute cord online:
Diameter of approximately 4.0mm (5/32”)
7 - 2 ply cabled inner nylon strands
Nylon 32 strand woven outer sheath
This tells me inside of the braid there are 7 individual nylon strands each having a 2-ply structure. I'm assuming that 32 individual strands of nylon are then used to braid the outer sheath.