Judas Iscariot
There are several explanations as to why Judas betrayed Jesus. In the earliest account, in the Gospel of Mark, when he goes to the chief priests to betray Jesus, he is offered money as a reward, but it is not clear that money is his motivation. In the Gospel of Matthew account, on the other hand, he asks what they will pay him for handing Jesus over. In the Gospel of Luke and the Gospel of John, the devil enters into Judas, causing him to offer to betray Jesus. The Gospel of John account has Judas complaining that money has been spent on expensive perfumes to anoint Jesus which could have been spent on the poor, but adds that he was the keeper of the apostles' purse and used to steal from it.
Kiss of Judas
The kiss of Judas, also known (especially in art) as the Betrayal of Christ, is how Judas identified Jesus to the multitude with swords and clubs who had come from the chief priests and elders of the people to arrest him, according to the Synoptic Gospels. The kiss is given by Judas in the Garden of Gethsemane after the Last Supper and leads directly to the arrest of Jesus by the police force of the Sanhedrin.