Wires used to conduct electricity, aka for electrical connections, are most often stranded wires. However, this is not the only option, there are two types of wires on the market; solid wire, and stranded wires. Solid wire, also called solid-core or single-strand wire, consists of one solid piece of metal wire. Stranded wire, the wires most of us are familiar with, are composed of a bundle wires which make a larger conductor cross section. The 'cross section' is the area of pure copper if you were to look at a perpendicular cut of the wire.
Stranded wires are more flexible than solid wire of the same cross-sectional area, but solid wires are cheaper to manufacture. Therefore, when selecting wires, the cheaper wires often have thicker strands inside them, of the same general gauge size. A stranded wire will have higher resistance than a solid wire of the same diameter since the cross-section of the stranded wire is not the same size. Stranded wire consists of circular wires, bound together, and you can't pack circles perfectly on top of one another. The smaller the wires,the tighter the fit, but there are unavoidable gaps between the strands. A stranded wire with the same cross-section of conductor as a solid wire is said to have the same equivalent gauge but is of a larger diameter to compensate for this area.
Why use stranded wires at all?
Stranded wires are selected for applications where higher resistance to metal fatigue, flexibility, movement, vibration and or shock is expected. Solid core wires would not be able to move as required. Such situations include connections between circuit boards, automotive connections, marine connections, your headphones, and the TV cord. The rigidity of solid wire in any of these applications would end badly. The more individual wire strands in a wire bundle, the more flexible, kink-resistant, break-resistant, and stronger the wire is. But more strands increase costs. For application where the wire moves, 19 is the lowest number of strands that should comprise the wire, 49 is an industry accepted minimum for many applications. The 7 strand wire minimum should only be applied to applications where the wire is placed and then does not move. For applications with constant repeated movement, such as assembly robots, vehicles, and headphone wires, 70 to 100 strands isn't a good idea, it's mandatory.
For applications that need even more flexibility (welding is the usual example), even more strands are used. One example is a 2/0 wire made from 5,292 strands of #36 gauge wire. The strands are organized by first creating a bundle of 7 strands. Then 7 of these bundles are put together into super bundles. Finally 108 super bundles are used to make the final cable. Each group of wires is wound in a helix so that when the wire is flexed, the part of a bundle that is stretched moves around the helix to a part that is compressed to allow the wire to reduce it's own stresses.
At high frequencies, current travels near the surface of the wire because of the skin effect, resulting in power loss over the conductive wire. Stranded wire might seem to reduce this effect, since the total surface area of the strands is greater than the surface area of the equivalent solid wire, but ordinary stranded wire does not reduce the skin effect. The skin effect is negated due to the short-circuit between all touching strands, which behave as a single conductor. However, for many high-frequency applications, proximity effect is more severe than skin effect, and in some limited cases, simple stranded wire can reduce proximity effect.