Structure is a generic term that refers to underwater habitat. Structure types include rocks, tree stumps, underwater cliffs, sudden depth changes, standing trees, fallen trees, bends in underwater channels, etc. Both sunfish and crappie prefer to associate with structure once they have finished spawning.
Structures is a generic term for artificial structure created by anglers or fisheries managers to improve fishing. These typically include brush piles, Christmas trees, chunks of old cement and concrete, old tires, and wooden “cribs.” In marine areas, artificial reefs made from sunken ships are artificial structures.
Finding and fishing in and around structure of any kind is a good strategy for catching fish.