> According to Hindu scripture, the Tulsi plant was a woman named Vrinda
> (Brinda), a synonym of Tulsi. She was married to the demon-king
> Jalandhar. Due to her piety and devotion to Vishnu, her husband became
> invincible. Even god Shiva, the destroyer in the Hindu Trinity could
> not defeat Jalandhar. So Shiva requested Vishnu - the preserver in the
> Trinity - to find a solution. Vishnu disguised himself as Jalandhar
> and tricked Vrinda. Her chastity destroyed, Jalandhar was killed by
> Shiva. Vrinda cursed Vishnu to become black in colour and he would be
> separated from his wife. Thus, he was transformed into the black
> Shaligram stone and in hisRama avatar, his wife Sita was kidnapped by
> a demon-king and thus separated from him. Vrinda then driwned herself
> in the ocean. Eventually Jalandhar was killed by Shiva. The gods or
> Vishnu transferred her soul to a plant, henceforth which was called as
> Tulsi.

>Another minor legend narrates that Lakshmi - the chief consort of Vishnu - slew a demon on this day and remained on earth as the Tulsi plant.

>(Source:[Wikipedia)][1]
> 
> According to Vaishnavas, it is believed in Puranas that during Samudra
> Manthana when the gods win the ocean-churning against asuras,
> Dhanvantari comes up from the ocean with Amrita in hand for the gods.
> Dhanvantari (the divine medico) sheds happy tears and when the first
> drop falls in Amrita it forms Tulasi. (Source: [Wikipidea][2]).

That's what from the books but Tulsi is herbal plant and also used for medicinal purposes.

  [1]: http://en.wikipedia.org/wiki/Tulsi_Vivah
  [2]: http://en.wikipedia.org/wiki/Ocimum_tenuiflorum#In_Hinduism