Christianss (see below) regard the act of speaking in tongues, as a gift of God through the Holy Spirit -- one of the Gifts of the Spirit. However, what some Christians see as divinely inspired, others and those in the secular world may dismiss as glossolalia. There are accounts of tongues actually being a foreign language understood by others present, but not the speaker. These, presumably, would not be categorised as glossolalia. Other religions also use glossolalia as a component of worship.
From a linguistic point of view, the syllables that make up instances of glossolalia typically appear to be unpatterned reorganizations of phonemes from the primary language of the person uttering the syllables; thus, the glossolalia of people from Russia, Britain, and Brazil all sound quite different from each other, but vaguely resemble the Russian, English, and Portuguese languages, respectively. Linguists generally regard most glossalia as lacking any identifiable semantics, syntax, or morphology-- i.e., as nonsense and not as language at all.
In the New Testament, the book of Acts recounts how "tongues of fire" descended upon the heads of the Apostles, accompanied by the miraculous occurrence of speaking in languages unknown to them, but recognizable to others present as particular foreign languages. Not only their peers, but also anyone else in the room who spoke any other language, could understand the words that the Apostles spoke. The book of Acts described the phenomenon in terms of a miracle of universal translation.
This Biblical case exemplifies religious xenoglossia, i.e., miraculously speaking in an actual foreign language that the speaker does not know. Many conservative Pentecostal (and other) Christians maintain that if the glossolalia does not manifest an actual human language, then it is not a genuine manifestation of the Holy Spirit.
Other Christians hold that this religious glossolalia comprises, at least in some cases, bona fide language inspired by the Holy Spirit: utterances in a language usually unknown to both the speaker and to the listeners. Yet other Christians hold that that all, or almost all, modern glossolalia has bogus origins, neither divinely inspired nor language-based. Contemporary Christians believe much more readily that the original instances of Christian glossolalia, as reported in the book of Acts, exemplified bona fide instances of actual human languages.
Links:
Christian view of speaking in tongues
Tongues in the New Testament
Contemporary Christian Glossolalia
Some Christians have claimed that they have witnessed, or personally engaged in, soi-disant "speaking in tongues". These claims have particular importance in the Pentecostal and in the Charismatic traditions. The belief that the gifts of the Apostles (Acts 2) continue to persist in the modern world forms a fundamental point of Pentecostal doctrine.