Pentecost, also know as the Feast of Weeks was one of the feast of the ancient Judaism. In these verses we find that this had begun and the disciples were gathered together one again. Like nothing they had ever experienced a rush of wind comes through with a great noise and fills the house they were in. God had come to them in the form of the Holy Spirit. He rested on each of their head that were present in the form of a tongue-like fire. No one flame was bigger than another. Everyone was given equal amounts of God's gifts, plan, and mandate. That wasn't all they were given, they also began to speak in tongues. In this context Luke describes this as different dialects that were understood by others. I am not by any means an expert on this topic so please read these passages to better understand what speaking in tongues is and what the boundaries are.(1 Corin 12 & 14) God did all of this in His own time and it is an interesting thing that He brought this about during a festival where all the faithful from the corners of the world would join together. They became excited and while some joined in with them others just thought that they had had to much to drink.
In verse 14 Peter gives the first apology(defense) in the history of the church. In verse 15-21 he quotes what the prophet Joel had spoke on about the day of the Lord was at hand. As Peter begins to preach more people come if not only to see what the commotion was about. He tells them that Jesus was the Messiah they had heard about for so many years and that David had written about. Jesus was the one who they had arrested and crucified. When the people heard this they were hurt and wondered what they should do. Some not wanting to know walked away but for those who stayed Peter quickly gave them an answer. Repent and be baptized in His name.
Those who received what Peter had to say were immediately baptized. That day 3,000 people joined the church. They began to truly fellowship with each other along with selling everything they had to share with others in need. When was the last time we had that same heart for others that we would give up everything for our brothers/sisters in Christ. We are such a selfish generation.