Trout and Salmon both belong to the ‘salmonidae’ Family. (They also share the family with Chars, Graylings and several other types of fish).
The names trout and salmon are a human concepts used to group species of fish into predefined categories. Unfourntatly, the Salmonidae population is so diverse that there is a lot of overlap between ‘trout’ and ‘salmon’.
Likewise, there is not really anything that separates a ‘Char’ from a ‘trout’. Many of us even choose to call Brook Char a trout. Char, Trout, Salmon are just words.
There is no straightforward definition that can be used to always separate a trout from a salmon.
The primary difference between a trout and salmon
The main difference that separates trout from Salmon is that trout often survive spawning, while salmon typically spawn once before dieing. Although, some Salmon do survive the spawn and will spawn again the following year.
There are exceptions to this rule. Between 5-10% of Atlantic Salmon (Genus Salmo) do survive, and will spawn again the following year if their condition allows. Some Atlantic salmon have survived to spawn up to four times.
I have read in too many places that all species of Pacific Salmon (Genus Oncorhynchus) die after spawning. This is simply not true. Over in east Asia the Masou Salmon (Oncorhynchus masou) is the exception to that rule with some (preferentially dwarf males) surviving allowing them to spawn a second time the following year.
So, yes. Even some Pacific Salmon do survive the spawn.
Salmon predominately are anadromous, While trout do both
One behaviour characteristics that partly separates trout and salmon is where they prefer to spend their lives.
Both Salmon and trout start their lives in freshwater, Salmon, as they grow have a very strong instinct to migrate downstream, and out to sea where they reach maturity.
While trout, are much more likely to stay in the river or migrate down to a lake. In saying that, quite a few trout also make the journey out to sea to take advantage of the abundance of resources.
And if landlocked, Salmon can spend their entire lifecycle in freshwater.
Does Salmon and Trout taste different?
Environmental difference such as diet has a bigger influence on taste than their genetics. A trout or salmon, that has been out at sea predating on the same prey will have a flavor profile more similar to each other, than a trout or salmon raised in a farm or had lived their lives in a river.
So a salmonidae diet, and overall body condition is the biggest influence on how they taste.
It is also a similar story with regards to the Nutrition value of their flesh. A sea living fish will likely offer different nutrition benefits than river fish or farmed fish.
In some parts of the world, the majority of ‘Salmon’ sold retail is actually farmed Rainbow trout
If I have to rate which is the best tasting. I will say a wild searun fish tastes better than a wild river fish, that taste better than a farm fish.