I'm no expert, but I will present you Rambam’s opinion as best as I understand it:
1. Before Matan Torah any man or woman could “marry” by simple mutual consent. However, there was no binding contract or mutual obligations (no Kesubah). Likewise, the “marriage” could be dissolved by either party simply walking away from the relationship. (See Mishnah Torah, Kidusha, hilchos Ishus ch. 1, law 1)
2. After Matan Torah it became a positive obligation to marry a woman with Kidushin. (Ibid)
3. Before Matan Torah prostitution was allowed, but the Torah prohibits this activity as a negative commandment (punishable by lashes). According to Rambam (at least as some understand him) this prohibition only applies to relations that is not intended as a “marriage” relationship.
(According to Raavad the woman must be a prostitute that makes herself available to all men, and not someone simply having relations with one man. Rambam holds that even any intimacy outside of a relationship is prohibited as prostitution (znus).) (Ibid, law 4)
4. After Matan Torah this pre-Matan Torah marriage is called a Pilegesh—a marriage without Kiddushin or a Kesubah. It is a mutual “marriage” relationship with no formal or legally binding contract.
5. Only a king is permitted to have a Pilegesh. (Mishnah Torah, Shoftim, hilchos Melachim, ch. 4, law 4.) (Except a Jewish Maidservant marrying her master, or master’s son after Yiud.)
6. In the times of the Shoftim, however, many people took Pilagshim. This is in transgression of a positive commandment, but is not punishable (by lashes) because there is no negative prohibition.
7. If a pilegesh commits “adultery” she is permitted to remarry her husband. The adultery in affect dissolved the “marriage” and it may be resumed, although it is considered improper to take back an unfaithful mate. (See Shaalos Uteshuvos R. Avraham ben Harambam and Radbaz on Mishnah Torah, ibid.)
8. The children of a pilegesh, or the children of her adulterous affair, will not be mamzeirim (as today with children born out of wedlock). A woman is only an “Eishes Ish” after kidushin. (Ibid.)
9. I don’t know if a pilegesh has to cover her hair. My best guess is that I cannot imagine she has too, because there is no Kesubah. Immodestly is a breach of the Kesubah. Of course, if she wishes to be modest (between her and G-d, and her and her husband) she might cover her hair, but this is in no way legally binding.
