It's a Gift of the Spirit
http://www.biblegateway.com/passage/?search=1+corinthians+12:4-11
Some believe that it is an older gift, that is no longer witnessed/practiced, others believe it is still practiced. Mainly differences in denomination (i.e. Assembly of God and Church of God practice it). It is speaking in a language that is understandable only by interpretation. I believe it is considered to be the language we will all speak in Heaven? In any case, generally in a service during worship, some people may start to worship God by uttering in tongues (i.e. strange language sounding to anyone else) to themselves, in other case, there is "Speaking and Interpretation" which usually happens when someone starts to speak loudly in tongues to the congregation, then all is silent, then someone else speaks the interpretation of what was said. This is considered to actually be the Lord speaking to the Church congregation.
I grew up in an Assembly of God church, and this happened quite often. It is pretty serious if the tongues occurs without interpretation, as it is meant to be interpreted (unless personal uttering as mentioned above) I am now in Baptist church, and they do not practice it.
This falls into the category of, "It is not necessary for Salvation" and is somewhat a mystery. It often causes disagreements in the Church. There are many studies on it that you can find. It is important to note that in Corinthians, it seemed the gift was being abused (i.e. used more for pride than its intention...)
These are just some thoughts on the subject. It would be a good idea to look up a Bible study on it from a reputable source that you trust, and gain some more insight.
Hope this helped shed some light on the subject for ya! Odds are I will have even gotten some things wrong, or others will disagree

...such is the nature of the topic.