Speaking in tongues appears in the New Testament in two forms: speaking in languages one does not know, and speaking in a kind of “spirit language” when under the influence of the Holy Spirit.
At Pentecost, Jesus’ apostles were filled with the Sprit and began to “speak with other tongues” and could be understood by people from other nations (Acts 2:1–12).
But more often the New Testament refers to the second meaning of “speaking in tongues,” an “unknown tongue” as a way of prophesying and praising God.
Acts mentions this several time (Acts 10:46; 19:6), and Paul stated he had spoken in tongues many times.
In 1 Corinthians 12 we learn that the Christians at Corinth placed a high value on speaking in tongues.
Paul told the Corinthians that this was fine, but that it was only one of many gifts from the Holy Spirit, and not everyone should expect the same gifts.
Pentecostals and charismatic Christians place a high value on gifts of the Sprit, including speaking in tongues. See 327 (Pentecostals) and 328 (charismatics).
For more on gifts of the Spirit, see 148.