As you can read in our article 'What is speaking in tongues?' tongues can either be in a known language (in other words, a language that someone, somewhere on earth would understand) or an unknown language - a heavenly language that no one understands.
For anyone who speaks in a tongue does not speak to people but to God. Indeed, no one understands them; they utter mysteries by the Spirit. (1 Corinthians 14:2)
Paul makes clear the difference between tongues and prophecy when he writes that prophecy 'speaks to people' but that 'anyone who speaks in a tongue edifies themselves'.
This is why you will sometimes find us, either as individuals or as a group of believers (for example, on our 'Arise & Shine' live broadcasts) praying in tongues without much interpretation. We are edifying (building ourselves up) by praying in these unknown languages and we are speaking not God, not man.