Okay, if nobody will read the Greenbean link...
Mysids are a general term for the family Mysidae, in the order Mysida. They are both fresh and salt water (brackish normally), some species being both. Mysis is a specific genus within the family Mysidae, but still not a specific species and still not either fresh or salt water. The mysis genus has been, and is being, recategorized as more is found out about it, so specific species are hard for even scientists to identify.
That said, as far as your aquarium is concerned, mysis and mysids are the same thing. PE Mysis is a brand name, but still the same thing. Crustaceans in this category can change by season, harvest, etc. They're all basically little shrimp things that fish eat.
They are a pain to raise. They aren't cheap to buy live, but they're cheaper than raising them is worth, unless you have slave laborers.
Jeff