"Q" stands for the German word "quelle" which means "source." This is a hypothetical gospel that many scholars do believe actually existed. James M. Robinson writes extensively about it in his latest book called The Gospel of Jesus: In Search of the Original Good News -- a fantastic book by the way. He even has a modern translation of this Gospel in his book.
My understanding of it is that it was "discovered" when scholars were comparing the contents of the synoptic gospels Mark, Matthew and Luke. The consensus is that Mark is the oldest of these three. Matthew and Luke borrowed much of their gospels from Mark. However, there is much that Matthew and Luke have in common that is not found in Mark. When you look at these common elements what you get is a what seems to be a collection of many of the most important sayings of Jesus. Many scholars therefore believe that Matthew and Luke also borrowed from this common source, which has come to be known as the "Sayings Gospel Q" or the "Q Gospel."
When the Gospel of Thomas was discovered it seemed to validate the existence of Q, since Thomas is also a collection of sayings of Jesus. Neither Q not Thomas have any information about Jesus' life or the events that surrounded his death. They are simply collections of his sayings.
Of course, this is a controversial theory, but many of the most respected scholars feel it is true. There is a translation of it in James M. Robinson's book as well as in The Complete Gospels by the fellows of the Jesus Seminar.
Billy in Lousiana
"With all there is why settle for just a piece of sky?"
Barbra Streisand in "Yentl"