Sprang is a very old and specialized braiding technique where the braid warp is stretched and grows from both ends simultaneously, as opposed to normal or open-end braiding that only grows on one end. Just like other braiding/plaiting techniques, sprang is a weftless technique, i.e. the resulting fabric is formed by only the warp made before the weaving process starts.
The technique is known from Viking-Age Scandinavia and thus it is not surprising that the term sprang is of Scandinavian origin. In Swedish it is known as språng or språngat band. As far as I know, the word is related to the modern Swedish noun springa, a slot, and maybe to the verb spränga, to explode, to violently rip apart. All of these related to the open structure that is one of the main features of sprang.
In basic sprang the threads of the warp are interlinked. That is, neighboring threads get twined around each other, but never change places. When basic sprang is worked on a multicolor warp, vertical stripes are formed throughout, just because the warp threads never change place more than by one thread. If you have experience in basic inkle/rigid heddle weaving, you are already familiar with this type of patterning.
In plaited sprang the threads of the warp are interwoven. That is, they travel diagonally from edge to edge and then turn, so each individual thread ends up doing a zigzag. When plaited sprang is worked on a multicolor warp, the result is some kind of plaid with half and possibly also whole lozenges, depending on how the warp is set up. If you have done wide flat braids or so called fingerweaving, you are familiar with this type of structures and patterns. However, in most cultures fingerweaving is done as free-end braiding and that turns the pattern shapes slightly oblong, arrow-like, while in sprang the emerging shape is symmetrical (the same can be achieved by open-end braiding with a pillow and/or weighted bobbins).
As braiding is closely related to bobbin lace (which probably originated from the passementerie workshops making braided and woven trims), one can also create more lace-like structures, e.g. with holes (cf. knitted lace) or bobbin lace ground work like the torchon, brussels and rose path ground. As my interest in sprang mostly is limited to the period of 600-1600, I don’t have experience in these structures and have to refer you to e.g. Jules Kliot’s excellent book Sprang. Language & Techniques, by Lacis Publications (still readily available and not to expensive). The 2011 issue of Strands, the Braid Society yearly publication, also had an interesting article on sprang in the Americas and it contains a lot of information on officer belts made in sprang with hole patterning.