Using textured yarn is a great way to add interest to your knitting, by letting the yarn do all the work.
The terms gimp, knop, boucle, and slub refer to spinning multiple plies, where one of the plies is held more loosely as the yarn is spun, forming loops or gimps. The yarn is often held together with a “binder” especially if one of the plies is a roving.
If you are interested in watching how hand spinners create this yarn, search spinning boucle yarn on YouTube.
Here is an excellent reference with clear images describing these types of fancy yarns.
Yarns of this style in the Knit it Now store: