Pentecostals believe in the Trinity (God): the Father, the Son and the Holy Spirit. We believe that Jesus Christ came to this earth. Pentecostals believe that Jesus Christ came on this earth through the Holy Spirit working through Mary, so that He had a human and divine nature. Mary was a virgin and gave birth to Jesus Christ (who was 1 Person, 100% Man & 100% God). Pentecostals believe that Jesus died for everyone on the cross, so that everyone who calls on Him and puts his/her trust in Him, and turns away from all his/her sins, will be saved from his/her sins and is on his/her way to Heaven. This is not by works but through faith and by God’s grace. When one repents (the turning away from all sins and turning to Christ), the believer receives the Holy Spirit. The Holy Spirit comes and lives inside of Him. These are the basics of the Christian faith.
What makes the Pentecostal faith unique is that Pentecostals believe that there is still a baptism in the Holy Spirit available to the believer. At that moment the believer speaks in tongues (glossolalia, unintelligible sounds, the Holy Spirit speaks through the believer. Whether that speaking in tongues happens always, does not necessarily have to be dogmatically believed in all Charismatic or Pentecostal churches. I believe that the gift of speaking in tongues is available to every Christian but I don’t believe that this has to be the initial evidence of having received the Holy Spirit) and he/she can use this gift of God to build himself/herself up in his faith, to be a bold witness. Pentecostal Christians also believe that the gifts of the Spirit haven’t ceased (cessassionism, for example: John MacArthur, evangelical / Calvinistic Christians) but they believe that this just continues from the Book of Acts (a Book of the Bible) and that these gifts are available to the believer, in the present time (continualism, for example: Pentecostal Christians, Charismatic Christians).