A "strong, driving" wind filled the room where they were gathered, and tongues of fire came to rest on their heads, allowing them to speak in different languages so that they could understand each other - but Peter pointed out that it was only the morning, and said the phenomenon was caused by the Holy Spirit. The Holy Spirit also gave the apostles the other gifts and fruits necessary to fulfill the great commission - to go out and preach the Gospel to all nations. It fulfills the New Testament promise from Christ (Luke 24:46-49) that the Apostles would be "clothed with power" before they would be sent out to spread the Gospel.
He also told the people that the Jesus they crucified is the Lord and was raised from the dead, which "cut them to the heart." When they asked what they should do, Peter exhorted them to repent of their sins and to be baptized. According to the account in Acts, about 3,000 people were baptized following Peter's sermon. For this reason, Pentecost is considered the birthday of the Church - The apostles and believers, for the first time, were united by a common language, and a common zeal and purpose to go and preach the Gospel.