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They are distinguished chiefly by the fact that the brown trout is largely a freshwater fish, while the sea trout shows anadromous reproduction, migrating to the oceans for much of its life and returning to freshwater only to spawn. Sea trout in the UK and Ireland have many regional names including sewin (Wales), finnock (Scotland), peal (West Country), mort (North West England) and white trout (Ireland).

The specific epithet trutta derives from the Latin trutta, meaning, literally, "trout".

The lacustrine morph of brown trout is most usually potamodromous, migrating from lakes into rivers or streams to spawn, although there is some evidence of stocks that spawn on wind-swept shorelines of lakes. S. trutta morpha fario form stream-resident populations, typically in alpine streams but sometimes in larger rivers. There is evidence that anadromous and non-anadromous morphs coexisting in the same river can be genetically identical. In common usage, the name "brown trout" is often applied indiscriminately to the various morphs.

The brown trout is normally considered to be native to Europe and Asia but the natural distribution of the migratory forms may be, in fact, circumpolar. There are also landlocked populations far from the oceans, for example in Greece and Estonia.

The fish is not considered to be endangered although, in some cases, individual stocks are under various degrees of stress mainly through habitat degradation, overharvest and artificial propagation leading to introgression. Increased frequency of excessively warm water temperatures in high summer, attributed to global warming, cause a reduction in dissolved oxygen levels which can cause 'summer kills' of local populations if temperatures remain high for sufficient duration and deeper/cooler or fast, turbulent more oxygenated water is not accessible to the fish.

This phenomenon can be further exacerbated by eutrophication of rivers due to pollution - often from the use of agricultural fertilizers within the drainage basin.

Overfishing is a problem where anglers fail to identify and release mature female fish back into the system. Each large female removed can result in thousands fewer eggs released back into the system when the remaining fish spawn. Another threat is other introduced organisms. Over time, this leads to reduction of the population of adult fish in the areas affected

The wide variety of issues that adversely affect brown trout throughout its range, do not exclusively affect brown trout, but affect many or all species within a water body. Thus altering the entire ecosystem in which the trout reside. In small streams brown trout are important predators of macro-invertebrates and declining brown trout populations in these specific areas would affect the entire aquatic food web.

S. trutta morpha fario prefers cold (though in comparison with other trout, this species has a somewhat higher temperature preference of about 60-65 °F, or 15.5-18.3 °C), well-oxygenated upland waters, especially large streams in mountainous areas. 'Cover' (protection) is important to trout, and they are more likely to be found where there are submerged rocks, undercut banks, and overhanging vegetation to provide protection from predators, bright sunlight and associated high water temperatures.

Access to deep water for protection in winter freezes, or fast water for protection from low oxygen levels in summer are also ideal.

The brown trout is a medium-sized fish, growing to 20 kg or more in some localities although in many smaller rivers a mature weight of 1 kg (2 lb) or less is common. The spawning behaviour of brown trout is similar to that of the closely related Atlantic salmon. A typical female produces about 2,000 eggs per kilogram (900 eggs per pound) of body weight at spawning.

Brown trout can live to ages of 20 years. But as with the Atlantic salmon, there is a high proportion of death of anadromous males after spawning and probably fewer than 20% of anadromous female kelts recover from spawning. The migratory forms grow to significantly larger sizes for their age due to abundant forage fish in the waters they spend most of their lives.