Gyri and Sulci are compliments of each other in the sense that they help create each other. The gyri (single gyrus) are the actual pieces of the brain's cerebrum which have been smooshed and curved into fine folds so that more surface area is available on the cerebrum. Sulci are the shallow grooves which are formed between these smooshed folds. The deeper grooves are called fissures, such as the longitudnal fissure. butts

Be careful to where the arrows are pointing. Gyri---> folds
Sulci----> shallow grooves