In slang terminology, a Holy Roller is a Charismatic, a Pentecostal, or some other church attender who worships in an ecstatic, frenetic, or out-of-control manner. The idea behind the epithet is that believers of certain denominations engage in bizarre behaviors during their worship services, including falling down on the floor and rolling around in the aisles. The term Holy Roller has always been considered derogatory and is meant to be offensive.
The term Holy Roller was specifically coined to describe those in the Charismatic branch of Christianity. Holy Rollerism is the set of beliefs behind the ecstatic worship experience. Sometimes the word Holy Roller is used more loosely today to refer to anyone who believes the Bible is true, who claims to be an evangelical, or who talks about God in public.
While it is true that some practices within the Charismatic movement are unbiblical (e.g, holy laughter, slaying in the Spirit, being drunk in the Spirit, etc.), and there is a definite emphasis on emotionalism and subjective personal experience in Pentecostal churches, there is no place in the church for the use of disparaging names. Let the unsaved world do the name-calling—the term Christian was most likely a term of contempt when coined—but let not the people of God stoop to such tactics.