Fly Reels - some terminology:
Die cast reels - This a less expensive mass production process generally used in the production of less expensive reels. Molten metal (usually an alloy) is poured or injected into a mold. Die casting allows for creating shapes and forms that would not be possible with CNC machining. The down side to die casting is that the cast material is typically heavier and more brittle and thus more prone to breaking when dropped. Die casting also does not allow for the very tight tolerances that are preferred.
CNC or machined reels - this manufacturing process is used by the vast majority of today's top reel manufacturers. Most are made using a CNC (computer numerical controlled) machine and are made of aerospace grade aluminum bar stock. The parts are usually anodized to prevent corrosion by salt water. Reels made this way are generally more light weight and more expensive than die cast reels and are stronger and will, with proper care, will last a very long time.
Spring and Pawl drags - this is also known as a "click drag". It is a very simple mechanism that uses a toothed gear that is engaged by a "tooth-like" piece and, with the help of a spring, creates some resistance as line is being stripped from the reel. Some are slightly adjustable but even on the maximum setting do little when a fish is taking line from the reel.
Disc Drag - this type of drag systems is more modern and is preferred by most fly fishermen today. This works as a brake as it creates tension/resistance as the fish takes line.but does not significantly increase tension when line is being wound back on the reel. The are two basic types of disk drags; those that apply the pressure to the inside of the spool or those that apply pressure within a hub. Many different materials are used in disc drag systems. They include cork ceramics, teflon, carbon fibre and others. A good disc drag can apply as much as 20 lbs. or more pressure when line is being taken from the reel. There are either sealed or non-sealed drags. Sealed drags are water tight and require no maintenance whereas non-sealed drags may require some light maintenance such a cleaning periodically.
Arbor size - It has only been in very recent years that arbor size has become part of the discussion on fly reels. Historically, a reel or associated spool had a very small central diameter or "hub". Arbor size is typically associated with just the larger size reels. Arbor size is typically talked about as small arbor (traditional fly reels) or large arbor and then there is mid size. Here is the deal about arbor size...with a large arbor reel you can retrieve line 2-3 times fast than on a traditional size or small arbor reel. This becomes more important in situations where a fish has taken a lot of backing. You can retrieve all that line much quicker. Some large arbor reels also have a wider spool to help maximize line capacity.