There is hardly a more natural way to feed your aquarium animals, or to clean your tank, than with microcrustaceans. It's pretty safe to say that these little creatures, frequently referred to as "pods," have indeed revolutionized saltwater aquarium keeping. That's not to say that hobbyists didn't have pods until recently; various forms can show up as hitchhikers on live rock, for example. What's different now is that any home aquarist can obtain specific types to perform specific duties. This is especially important because not all pods occupy the same ecological niche, nor do they all do the exact same things in aquaria.
By far, the most commonly used pods in aquaria are copepods and amphipods. Copepods are assigned to the huge subclass Copepoda, which is divided into 10 orders. Amphipods belong to the subclass Eumalacostraca within the order Amphipoda. Copepods and amphipods share a lot of traits. They certainly might look a lot alike to untrained naked eye. To the point, they both contribute to tank cleaning duties and both may be eaten by fish, corals, etc. But that's where a lot of these similarities end.
Copepods and amphipods have slightly different impacts on their environment. Because copepods are much smaller, they clean more "deeply" by getting into tighter spots in the rock and sand. Amphipods are better at taking on the big jobs, being better at scavenging larger concentrations of waste (e.g. chunks of uneaten fish food). Copepods are mainly herbivores whereas amphipods are mainly detritivores. However, amphipods are not above eating any copepod they can catch! Finally, given their very different adult sizes, copepods and amphipods are selectively preyed upon by different aquarium animals.
The take? Marine aquaria (especially reef aquaria) are diverse environments. Thus, it's best to maintain a varied community of cleaner and feeder organisms. A wide variety of microcrustaceans helps to ensure that your tank is as clean and as well-fed as possible! And the best way to ensure pod diversity is to (1) provide ample hiding spaces (such as MarinePure) for smaller pods and (2) reseed often!