Reel bodies (often referred as the "housing") can be made of graphite or aluminum, or sometimes a combination of both. Aluminum housing is stronger than graphite and exhibits less flex; however, graphite is lighter. Deciding whether strength or weight is more important is ultimately a personal choice. The highest quality freshwater reels tend have aluminum bodies; however, if your chosen playing field is saltwater, graphite is the way to go due to its corrosion-resistant qualities.
Another factor to look for is a solidly constructed reel body. There should be no loose or flimsy parts, and all moving parts should be smooth in performance and have no back play whatsoever. Also, spinning reels have more parts than casting reels. Choosing a reel with fewer parts can reduce the chances of mechanical breakdown.