The phenomenon known as Speaking in Tongues is practiced mostly by those with a Pentecostal and Charismatic background. It is manifested when individuals speak, either in their private prayers or in a public meeting, in what sounds like an unknown language. Some Christians believe so strongly in this manifestation that those who display the ability to speak in "tongues" are viewed as being more spiritually mature than others - or may be viewed as not being converted at all.
The primary scripture used to justify this behavior is found in Acts 2. On God's annual holy day known as Pentecost, just ten days after Jesus ascended to heaven, the disciples were gathered together in Jerusalem when they first received the empowerment of the Holy Spirit. The most noticeable feature of this occasion was that each of the disciples spoke in known languages (other than their own), and those in the audience, who were from many other nations, were surprised to hear them speak in the language of their country.
6. And when word of this went out, the multitude came together and were confounded, because each one heard them speaking in his own language. (Acts 2)
If tongues speaking in done privately, like in prayer, it is considered a 'prayer language.' Those who manifest this behavior may not even know what they are saying. Those who tongues speak in public, during a religious gathering, are seen as delivering a message from God. The expectation is that alone with the message God will inspire to interpret it as well.
Those speaking in tongues usually do not claim that what they are saying is in a known human language. They believe they are uttering words in an unknown spiritual language (possibly an angelic language). Any interpretation offered regarding what is said is not seen as a literal but as a loose explanation of the message given by God's spirit.