From BiblePortal Wikipedia

Bridgeway Bible Dictionary [1]

When the Bible says that people spoke in tongues (‘other tongues’ or ‘strange tongues’), it means that their speech was in words that were not of their own language and that they did not understand, unless someone interpreted them. Beyond that simple definition, general statements about tongues become difficult, because of the different sorts and uses of tongues in the New Testament.

In the book of Acts

The birth of the New Testament church took place in Jerusalem on the Day of Pentecost, when about 120 disciples received the Holy Spirit as Jesus had promised ( Acts 2:1-4; cf.  Acts 1:4-5). On that occasion the disciples spoke in tongues that people from other linguistic groups understood as their native languages ( Acts 2:4-11).

There are only two other places in Acts where the writer records that people spoke in tongues, but in neither case is it clear whether the tongues were languages already in use or something completely different ( Acts 10:44-46;  Acts 19:1-6). On each occasion there seems to have been a special reason for the people’s speaking in tongues, as each case is a departure from what had been normal till that time. The speaking in tongues was a striking outward and visible demonstration that the people concerned had received the Holy Spirit and were introduced into the church the same as the original disciples were on the Day of Pentecost. (See also Baptism With The Spirit .)

In Paul’s letters

Tongues that were spoken in the normal meetings of the church seem to have had a different purpose. They were a gift that the Holy Spirit gave to certain people to exercise in their praise to God ( 1 Corinthians 12:10;  1 Corinthians 12:30;  1 Corinthians 14:2). People were to use the gift publicly only if someone could interpret the words in the normal language of the worshippers, so that all present could benefit. This indicates that whereas the tongues referred to in Acts were irresistible, those referred to in Corinthians were under the control of the speaker ( 1 Corinthians 14:13;  1 Corinthians 14:27-28). Also, those who spoke in tongues in the church were to do so one at a time, and no more than two or three in all ( 1 Corinthians 14:27).

It seems that the languages spoken in these cases (i.e. in the church) were different from any known languages. The Christians at Corinth, still influenced by attitudes from their former idolatrous days, were apparently impressed by these tongues, and considered that those who spoke them were spiritually superior. However, the situation got out of control, and people made some unusual, even blasphemous, statements. According to Paul, this was evidence that those who spoke in tongues were not necessarily speaking by the Holy Spirit ( 1 Corinthians 12:1-3; cf.  1 John 4:1-3).

Although Paul allowed the gift of tongues, he was cautious in encouraging people to seek it. He encouraged them rather to seek those gifts that proclaimed God’s Word and consequently built up the hearers ( 1 Corinthians 12:28-31;  1 Corinthians 14:3-5). Any speaking that took place in the church had to have meaning to the audience ( 1 Corinthians 14:6-12;  1 Corinthians 14:19). It had also to have meaning to the speaker, for he was not likely to be spiritually built up if he did not understand what he was saying ( 1 Corinthians 14:13-15).

The Corinthians’ concern for the spectacular demonstrated their immaturity, and their misuse of tongues brought dishonour on the church ( 1 Corinthians 14:20-25). Like all the gifts of the Spirit, the gift of tongues was given to only some in the church, and it could be wrongly used or falsely copied ( 1 Corinthians 12:3;  1 Corinthians 12:7;  1 Corinthians 12:10;  1 Corinthians 12:30;  1 Corinthians 13:1). Paul therefore emphasized that the evidence of the Spirit’s work in people’s lives was not whether they spoke in tongues, but whether their lives displayed the fruit of the Spirit. And the fruit of the Spirit is Christlike character ( Galatians 5:22-23; see Gifts Of The Spirit ).

People's Dictionary of the Bible [2]

Tongues, Gift of.  Joel 2:28;  Acts 2:16;  Mark 16:17; comp.  Matthew 10:19-20;  Mark 13:11. This gift was of two kinds. The first gift was the power to "declare the wonderful works of God" in languages ordinarily unknown to the speakers, for the instruction of foreign hearers.  Acts 2:4-11. The other form of the gift of tongues is thought to have been an ecstatic form of worship, chiefly praise, but requiring interpretation.  Acts 10:46;  1 Corinthians 12:30.

Charles Buck Theological Dictionary [3]