Pentecostals
Pentecostals are a rather well known Christian denomination, however they are not an organized church, despite being considered a denomination. They do not have a set creed or group of beliefs, and the belief structure tends to change, sometimes drastically, from congregation to congregation - Though there have been church groups within the Pentecostal 'movement' as it is called. Most, though again not all, are believers in the Secret Rapture belief, the 7 year tribulation in which God's people are secretly raptured away suddenly to avoid the time of trouble, while the 144,000, usually seen to be literal Jews, stay on earth to preach a second chance to those left on earth. Pentecostals are the most typically called "Charasmatic" churches, known by most of the Christian community for their rather loud worship displays, pounding music in some congregations, others babbling profusely, as they believe it is evidence of the Holy Spirit by way of speaking in tongues, or loud on stage healings. Though I have been told that Charismatic is a movement within a movement, one of the said church groups within the Pentecostal denomination.
The actual beliefs tend to vary as stated, so it is difficult to give an actual summation of their beliefs, since, as a whole, they do not have a shared set. Most believe in a sunday Sabbath, eternal hellfire, and baptism by whatever method desired. Some Pentecostal churches are quiet and reserved, many are loud and near riotous, and the worship services can differ as much from congregation to congregation as the beliefs themselves.
The name "Pentecostal" is a reflection on their "tongue speaking" habits, a commemoration of the day of Pentecost in the book of Acts, where the Holy Spirit came on the disciples as fire, and some of them began to speak in tongues, languages unknown previously to them, to men who were watching them in their native tongues.
The difference between the book of Acts and the Pentecostal belief is that the Apostles spoke in actual languages, whereas Pentecostals believe that it is a holy language, in which not even you, but the Spirit and God alone (They are Trinitarian in all churches I have known) know what you are praying.