Global Line in Egmont Højskolen,Denmark/2006.

Stuttering is a complex set of behaviours that may involve repeating sounds, syllables or words, prolonging sounds, blocking or hesitating, and avoiding or substituting words.
There may be other secondary behaviours associated with stuttering such as excessive muscle tension in the face, neck, back or stomach. Distortion of the face can occur with grimacing, frowning, etc. There may be unusual movements of the head or limbs. There are as many different patterns of dysfluent behaviour as there are people who stutter.

Stuttering, especially in adults, can be accompanied by emotional embarrassment, distress and frustration. People who stutter often avoid speaking situations, because of the associated fears and distress, e.g. talking on the telephone. People who stutter may avoid choosing a career in which they believe their stuttering will become obvious or be a handicap.
Stuttering usually starts in childhood between the ages of two and five. About 5 per cent of children under the age of five will experience dysfluent speech while learning to talk and about a quarter of them develop chronic stuttering. Stuttering is more common in boys than in girls. The ratio is about 3:1 or 4:1.

Stuttering varies in degree of severity and according to the situation. It is episodic. For example, a child may be fluent for days or weeks at a time then dysfluency can reappear. Adults commonly report "good" days and "bad" days.
It is well known that some speaking conditions reduce stuttering severity. Stuttering will be reduced or eliminated when the person is singing or speaking in a chorus, but stuttering will reappear as soon as the situation changes.