Many children up to about the age of five will run into problems with stuttering. This is a completely normal stage of development and most children will outgrow it. For the ones who do not, stuttering can have an ongoing, profound affect on their lives.
Developmental stuttering is by far the most common type and begins in childhood and doesn't go away, continuing on into adulthood.
Neurogenic stuttering appears later in life, in a person who had no previous problem with it. It can be caused by a head injury or a stroke, or some other brain trauma. This type of stuttering is a relatively rare occurence.
If your child stutters, you can help them deal with it by being supportive and relaxed when they speak. Don't finish their sentences for them or display impatience or disapproval.
To make things more comfortable for friends who stutter, give them plenty of time to say what they are trying to say. Suggestions like "Relax," are in reality not relaxing. This just makes the afflicted person feel more self-conscious, and more pressured. And the outcome will probably be even more pronounced stuttering than before.
Some people who stutter find some relief from it in a few different activities. Some don't stutter when they sing, or recite. I was in a play years ago with a friend who stuttered. Except when he was saying his lines for the play. Then he had a beautiful speaking voice.
It may be helpful to see a speech-language pathologist (SLP). These are specialists in speech, language and voice disorders. For young children, early intervention with an SLP may help prevent some of the potential difficulties. But at any age, seeing an SLP may be helpful. They can provide different types of therapies which help the individual control their speaking patterns and habits.
Stuttering can sometimes be decreased by speaking slower, and by more controlled breathing. This also can help to ease the anxiety that so often occurs when a stutterer finds himself in a situation where he must speak. Reduced anxiety can sometimes reduce the tendency to stutter.
Stuttering is not a sign of lack of intelligence.