Orpheus was a mystical drug that was highly potent and intoxicating. It was used mostly by demons but humans used it as well. In the case of a human, they got high and vampires fed on them to get the same effect. The effect was as if the user was high on heroin or another opiate. Too much Orpheus could result in a coma or even death.
The drug only caused a high in humans after they were bitten by a vampire, with the subject operating at his or her apparent physical norm until this occurred.
The drug had such a negative reputation in the supernatural world that Lorne once banned a group of demons from Caritas because they were dealing the drug. It was the only occasion he ever felt obligated to ban anyone from the club.
Orpheus was used by Faith Lehane to capture Angelus, Faith pocketing a sample when she witnessed someone high on the drug and then injecting herself while Angelus was distracted during a subsequent fight. While under the drug's influence, the two went on a spiritual "walk" through Angel's memories as Willow Rosenberg prepared to release his soul into the ether and perform the Ritual of Restoration, culminating in Angelus facing the remnants of Angel in his psyche before Willow restored the soul and Angel regained control of his body.