Reels are made in a variety of configurations and sizes that match your rod weight (wt) and fishing conditions. The first variance is the arbor. The arbor is the center of the reel that the line is wrapped around. There are standard (small), mid, and large arbor reels. The difference is that with every revolution of the spool, x amount of line is wrapped back on the reel.
With standard arbors you retrieve 2 to 4 inches of line per revolution. The mid and large arbor reels can pick up 4 to 10 inches of line depending on the size of the reel. The reason this matters is that in some applications, it is imperative to get the line back on the reel fast. A large arbor reel would be well suited for picking up line faster.
The next thing to think about in looking at a reel is the drag system. There are a lot of different systems out there, but they all boil down to 3 basic types- click and pawl, disc, and cork. The whole purpose of a drag system is to create resistance against the fish making it harder for it to pull line off the reel. Click and pawl reels have a very limited amount of resistance, but resistance can be increased by adding a heavier spring if it is available from the manufacturer. Disc drag systems have much more resistance and adjustability. Cork drags also have more resistance and adjustability as well as little to no maintenance. Good reels are made of aluminum or other corrosion resistant materials. Plastic or composite reels will get the job done, but are not made for longevity or durability.