The SPD cleat is probably the most familiar cleat in the cycling industry. It is an unchanged staple that simply put, works. Shimano has made a few versions of these that have slightly different characteristics, so make sure you choose what will work best for you.
The SH-51 Single Release cleats are the stock replacement cleats for all Shimano SPD mountain pedals. Perfect cleat for all SPD mountain pedal users that want the best performance and retention. For any experienced rider retention = safety. The steel constructions makes them very durable and include washers and mounting hardware.
Shimano also offers what they call - Multi Release, meaning the cleats will release in multiple directions. The idea is that the cleats focuses on ease of release instead of retention and makes it extremely easy for riders to unclip from their Shimano pedals. We generally recommend these cleats for riders just getting used to clipless pedals or folks that have had issues in the past with not being able to get out of their pedals. On that note, we do not recommend them for more experienced riders or those looking for a cleat that will offer good retention on rough terrain.