(covers information from several alternate timelines)
Hair is a series of filaments of dead cells that grow on the skin of animals, usually mammals. In humanoids, hair often grows on the top of the head, where it is styled for cultural reasons. On mammals, it was generally termed "fur".
Individuals could be described in terms of their hair color, such as "blond(e)" (yellow/gold) (TOS: "Where No Man Has Gone Before", "Shore Leave"; TNG: "Too Short a Season") and "brunette" (brown) (TNG: "The Big Goodbye", "The Quality of Life")
A thirteen year old Beverly Crusher changed her hair color from red to brunette, but dissatisfied with the result, quickly changed it back. While playing poker in William T. Riker's quarters in 2369, Beverly related this after Riker said he'd like to see her as a brunette, which piqued his curiosity further. She then agreed that changing her hair color would be her forfeit if one of the other players won. (TNG: "The Quality of Life")
In 2369, Jadzia Dax told Kira Nerys about Morn who was asking her for dinner. She declined but though that he was cute because of his seven or eight wiry hairs sticking out of his forehead. (DS9: "Progress")
When Lwaxana Troi asked Constable Odo about his hair, he told her that this was no real hair but a part of himself, the shapeshifter. She then asked curiously if he studied hairstyles, to which he replied that he imitated the style of the Bajoran who studied him. (DS9: "The Forsaken")
Notably without hair, the Bolian race have sometimes been seen to wear a toupée to hide this fact. (DS9: "The Muse") Bolian barbers included Mot and V'Sal. (TNG: "The Host", "Data's Day", "Ensign Ro")
When Seven of Nine was freed from the Borg Collective in 2374, The Doctor was able to stimulate her hair follicles to re-grow. (VOY: "The Gift") This was done at least once more, when, in 2376, Lyndsay Ballard returned to the crew of the USS Voyager as a Kobali. (VOY: "Ashes to Ashes")