"Haar" is a common or a neuter noun, depending on the meaning. Note that "haar" can also be a possessive or personal pronoun, see 3. and 4. below.

1. hair
[noun]
[het haar, <no plural>]

You say "het haar" when talking about hair in general (such as body hair) or your haircut/style/do.

Examples:
– "Mijn haar ziet er niet uit, sorry, maar zo kan ik echt niet uit." 
("My hair looks really bad, I’m sorry, but I can’t possibly go out like this.")

– "Wat zit je haar leuk!" – "Nou, dankjewel!" 
("Your hair looks really nice!" – "Well, thank you!")

– "Sander heeft gisteravond zijn haar gemillimeterd." 
("Sander trimmed/cropped his hair last night." Here the verb "millimeteren" is obviously derived from the length of the hairs left after trimming.)

– "Haar haar zit leuk, vind je ook niet?" 
("Her hairstyle is really nice, don’t you think?")

– "Is jouw haar geverfd, of is dat je echte haarkleur?" 
("Is your hair dyed, or is that your real hair colour?")

Expressions:
– "Als je haar maar goed zit…": literally "as long as your hair looks fine…" this says something like "why worry over something…"

Related words:
– Kapsel: hairstyle/do [noun] [het kapsel, de kapsels].
– Haarstijl: hairstyle [noun] [de stijl, de stijlen].
– Borst/been/schaam/okselhaar: chest/leg/pubic/armpit hair [noun] [het haar, <no plural>].
– Kaal: bald [adjective].

2. hair
[noun]
[de haar, de ha-ren]

You say "de haar" when talking about a single hair, or "de haren" in case you mean a collection of single hairs as opposed to a hairdo.

Examples:
– "Van al dat harde werken krijg je grijze haren als je niet oppast." 
("All that hard work may cause grey hairs (to appear) if you don’t watch out.")

– "Van wie zijn die blonde haren?" – "Ik heb geen idee, echt waar schat…" 
("Whose blonde hairs are those?" – "I haven’t got a clue, honestly darling.")

– "Er zit een haar in mijn salade." – "Bah, wat vies!"
("There is a hair in my salad." – "Yuck, how disgusting!")

Expressions:
– "Daar krijg je grijze haren van": that will turn your hair grey.
– "Op een haar na": very nearly.

Example:
– "De auto raakte de voetganger op een haar na."
("The car very nearly hit the pedestrian.")

– "Geen haar op mijn hoofd die daar aan denkt": I would not dream of it. Lit.: "Not a single hair on my head that will think of that".

Related words:
– Behaard: hairy [adjective].

3. her
[possessive pronoun]

Example:
– "Ik zweer het, dat is mijn haar, niet haar haar!" 
("I swear, that is my hair, not her hair!")

4. her
[personal pronoun]

Example:
– "Ok, ik zal het haar zeggen." 
("OK, I will tell her.")