Hair
Hair was a series of filaments of dead s that grew on the skin of animals, usually mammals. In humanoids, hair often grew on the top of the head, where it was styled for cultural reasons. 

The Krenim Annorax kept a lock of hair in a glass globe aboard his weapon ship. It was from his wife. ()

Physiology 
Similar to hair, and generally found on mammals, was much finer fur or much heavier wool.

Color 
Common hair colors included blonde, brunette, red, (raven) black or dark, and gray.

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. ()

In 2371, upon Kathryn Janeway claiming to be from Kalto Province, Terla remarked that she'd never seen a Kalton with her color of hair. Janeway replied, "Well, now you have." ()

In 2376, the assimilated Brunali Icheb recognized his mother's hair color, prompting  to threaten him with deactivation for wasting time on irrelevant matters. ()

In the anti-time future, Data dyed half of his hair grey to look older, a fashion choice Jessel abhorred, believing he resembled a skunk. ()

Baldness 
Notably without hair, the Bolian race have sometimes been seen to wear a toupée to hide this fact. () Bolian barbers included Mot and V'Sal. ()

The Federation Captain Jean-Luc Picard had been bald for most of his adult life. (; ) Guinan was attracted to bald men. ()

In 2369, Jadzia Dax told Kira Nerys about Morn who was asking her for dinner. She declined, but thought that he was cute because of the seven or eight wiry hairs sticking out of his forehead. ()

When Seven of Nine was freed from the Borg Collective in 2374, The Doctor was able to stimulate her hair follicles to re-grow. () This was done at least once more, when, in 2376, Lyndsay Ballard returned to the crew of the  as a Kobali. ()

Members of Dala's species tended to be bald, but could easily wear wigs to fake having hair. Despite this, at least some found it uncomfortable. ()

Care and styling 
Scissors were used to cut hair; one who cut hair as their profession was known as a barber. ()

Just prior to and during the Federation-Klingon War of 2256, Klingons appeared without hair en masse. It was only after the war concluded in 2257 that the Klingons grew their hair back. ()

In the alternate reality, 's hair was curly, instead of straight like his prime reality counterpart. (; ; )

After being made into a receptacle for the negative emotions of Ves Alkar, Counselor Deanna Troi began sporting a series of increasingly elaborate hairstyles as possessive feelings of jealousy were kicked into overdrive. ()

According to Mot, he found Klingon hair "thick and luxurious" and "a pleasure to cut". He suggested that Worf use a conditioning agent on his. ()

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. ()

External links 
 
 
 
 

de:Haar
Anatomy