Hero skill trainers are a special type of skill trainer found in Guild Wars Nightfall and Guild Wars Eye of the North. In Nightfall, they have the tag "[Hero Skills]"; in Eye of the North, they have custom tags (such as "[Asuran Talent Scout]" for Lexx).
Learning a skill from a hero skill trainer costs 1 hero skill point. The skill will then be known by the character and will be unlocked on your account. Hero skill trainers can teach skills from any profession, regardless of the character's current professions; they will be available upon switching to the appropriate secondary profession.
Like ordinary skill trainers, hero skill trainers will offer all skills that the character does not know, including skills that are already unlocked on your account. Unlike ordinary skill trainers, however, they do not indicate whether a skill is unlocked or not.
For a list of all known hero skill trainers see the hero skill trainer locations page.