For a quick overview, here's an article by the BBC to give you an idea what the Pentecost is:
PentecostWhat is Pentecost? Pentecost is regarded as the birthday of the Christian church © Pentecost is the festival when Christians celebrate the gift of the Holy Spirit. It is celebrated on the Sunday 50 days after Easter (the name comes from the Greek pentekoste, "fiftieth").
It is also called Whitsun, but does not necessarily coincide with the Whitsun Bank Holiday in the UK.
Pentecost is regarded as the birthday of the Christian church, and the start of the church's mission to the world.
The Holy SpiritThe Holy Spirit is the third part of the Trinity of Father, Son, and Holy Spirit that is the way Christians understand God.
Celebrating PentecostPentecost is a happy festival. Ministers in church often wear robes with red in the design as a symbol of the flames in which the Holy Spirit came to earth.
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Pentecost SymbolsThe symbols of Pentecost are those of the Holy Spirit and include flames, wind, the breath of God and a dove.
The first PentecostPentecost comes from a Jewish harvest festival called
Shavuot.
The apostles were celebrating this festival when the Holy Spirit descended on them.
It sounded like a very strong wind, and it looked like tongues of fire.
The apostles then found themselves speaking in foreign languages, inspired by the Holy Spirit.
People passing by at first thought that they must be drunk, but the apostle Peter told the crowd that the apostles were full of the Holy Spirit.
Pentecostal ChristianityPentecost is a special day for any Christian, but it is emphasised particularly by Pentecostal churches. Pentecostal Christians believe in the direct experience of the Holy Spirit by believers during all of their services.
http://www.bbc.co.uk/religion/religions/christianity/holydays/pentecost.shtml