They are all related, but some are closer than others. Members of this group are found around the world, not only in streams and rivers, but also in lakes and oceans.
Salmonids tend to prefer cool temperatures, and they can be found even in the Arctic Ocean.
Whitefish, ciscos, and graylings are also salmonids. Often, the main factor in whether trouts migrate to sea seems to be whether the coast is near enough to their home streams. They will begin small, with microscopic cladocerans and copepods, and build up as they grow to aquatic invertebrates like flies and fly nymphs. That is why some salmonids have such bright orange meat, including rainbows, Pacific salmon, and Atlantic salmon. Sockeye salmon also have an orange tint to their flesh.