Nectar is a drink consumed by the gods. In mythology, it gives immortality to whoever drinks it, but it does not in the Percy Jackson and the Olympians series. It is closely related to Ambrosia (for healing purposes). Nectar can be drank in small amounts by wounded demigods.
Nectar is used as a healing drink when someone is injured. Demigods can drink it, but if they have too much of it, they are in danger of being incinerated. Humans can't drink it at all without being incinerated. It has been implied that nectar tastes like anything the drinker likes (favorite food or drink). This is shown in The Lightning Thief, when after recovering from the battle with the Minotaur, Percy Jackson is surprised to find that the nectar tastes like his mother's blue chocolate chip cookies.
Nectar does not appear in The Lightning Thief film, although Percy is given nectar by Annabeth in a deleted scene. Its flavor is never mentioned, although Percy states that it's 'amazing'. Also, Annabeth mistakenly calls it the 'food' of the gods instead of the drink of the gods.