There is no one "pentecostal" church, but here are beliefs many of these churches have in common, even if one specific church may not believe in all of them:
* Strong emphasis on glossolalia ("speaking in tongues") and other "gifts of the Holy Spirit" such as laying on of hands leading to healing. Belief that all Christians have these gifts and a repudiation of the mainstream belief that they were a temporary gift miraculously given to the apostles and ministers of the early Church for the purpose of spreading Christianity faster.
* Glorification of emotion over reason. Feels = reals. Sermons and revivals centered around causing emotional reactions in churchgoers. Many revivals consist of videos that are meant to arouse emotion in viewers, such as graphic depictions of the passion of Christ leading to crying, or dramatic videos of missions leading to joy, et cetera. That's not to say Pentecostals don't use what they see as rational theological arguments, but rational argumentation takes a backseat to emotion.
* Repudiation of once saved always saved. Belief that works are required for salvation. Belief that if a believer doesn't endure to the end, they never actually even got saved. Dim view of backslidden Christians.
* Creative interpretations of the trinity, bending the Nicene creed to the breaking point or even breaking it completely. Modalism, unitarianism, et cetera are common in Pentecostal denominations.