"Pentecostalism has become the fastest growing segment of Christianity. "It is growing at a rate of 13 million a year, or 35,000 a day. With nearly a half billion adherents, it is, after Roman Catholicism, the largest Christian tradition" (Christian History, "The Rise of Pentecostalism," issue no. 58, vol. XVII no. 2, p.3). In addition, the largest church in the world (the Yoi Do Full Gospel Church) is a Pentecostal church in Korea, pastored by David Yonggi Cho, with a weekly worship attendance of 240,000. Two Pentecostal Churches in Buenos Aires attract together 150,000 each week (ibid.)."
The above is the opening paragraph from the very interesting article by Gary E. Gilley whose link follows. I, personally, had no idea this movement was as strong as it is. There are a number of denominations which comprise the movement, but even so, I did not know there were that many. Here are some of the largest and best known denominations among them: Church of God in Christ, International Church of the Foursquare Gospel Church of God (Tenn.), Church of God of Prophecy, Pentecostal Holiness Church, Fire-Baptized Holiness Church, Pentecostal Free-Will Baptist Church, The Assemblies of God and The United Pentecostal Church.
Several of these are quite large within themselves. For instance, the Church of God in Christ, and the Assemblies of God have an enormous number of adherents. In all 50 states and abroad.
The prime reason for these people being referred to as Pentecostals is because most, if not all, profess to have the power to speak in tongues as did the apostles on the Day of pentecost. We know however that the apostles were given that power miraculously so they could speak in the many languages that were present there with them that day. So they could be understood by all. You need to read this very informative article. It is not too long and excellent reading.