What is the Sunnah concerning a woman's hair, and must it always remain parted to adhere to the Sunnah? And what exactly is the description of this parting?

There is no specific hairstyle commanded by the Prophet (peace be upon him), as this falls under customs. It is permissible for both men and women to style their hair in the way they prefer, as long as there is no other impediment, such as imitating disbelievers.

It is permissible for a woman to cut, part, and style her hair if her intention is free from imitating disbelieving women, harlots, or men, and if she intends permissible adornment for her husband or to lighten the burden of her hair and facilitate its styling. The evidence for this is that the wives of the Prophet (peace be upon him) would cut their hair until it was like a wafra (hair reaching the earlobes), as mentioned in Sahih Muslim from the hadith of Abu Salama bin Abdul Rahman bin Awf. They would do this to lighten the burden of their hair. Al-Nawawi said in his commentary: "This contains evidence for the permissibility of women lightening their hair."

Regarding letting hair hang down (sadl) and parting it (farq), in the two Sahih collections, and the wording is Muslim's, from Ibn Abbas (may Allah be pleased with them both), he said: "The People of the Book would let their hair hang down, and the polytheists would part their hair. And the Messenger of Allah (peace be upon him) loved to agree with the People of the Book in matters regarding which he was not commanded. So the Messenger of Allah (peace be upon him) let his forelock hang down, then he parted it later." Al-Nawawi said: "The conclusion is that the correct and chosen view is the permissibility of both sadl and farq, and that farq is better."

However, the Muslim woman must wear a full hijab, and it is not permissible for her to display her adornments or her hair in front of non-mahram men. Rather, it is permissible to uncover it only for her husband and her mahrams (unmarriageable kin).