If you look up the definition of stuttering (also called stammering), you will find slight variations depending on which book you read or website you visit. This is because the exact cause of stuttering is still unknown. But all experts agree that stuttering is a communication disorder in which the flow of speech is involuntarily disrupted.
We all try to cope with these "disruptions," but the research shows that tension can increase over time. These speech disruptions often are combined with physical tension that develops over time due to an effort to speak.  This tension can become more pronounced and present externally as tremors and rapid oscillations of the face, lips, or jaw.  This tension can also include other behaviors such as hitting the spastic motions (i.e., slapping or hitting of the limbs against one another), moving heads, stamping feet, closing eyes, jerking necks, clenching fists, etc. These responses, as Parry states in Understanding & Controlling Stuttering, are the different reactions we develop as we try overcome our primary obstacle, our speech. 
Though stuttering can present in adults, usually stuttering begins in childhood between the ages of 2 and 5. About 5% of children will stutter while developing speech. Most of those children will resume normal speech by age 12, but about 1% will continue with a lifelong struggle with this disability.  That equates to about 3 million people in the United States and 68 million worldwide. 
Now that we know what stuttering is, let's discuss what stuttering is not.
- Stuttering is not a mental disorder. It is a physical condition. It is a disorder that manifests differently for everyone. People who stutter know exactly what they want to say; they just have trouble with the execution.
- Stuttering is not constant. It is actually a variable condition. A person who stutters can have extended durations of fluent speech, broken up by periods of disfluency. There are also varying levels of severity.
- Stuttering is not a choice. People who stutter do so involuntarily. It is a condition that leads sufferers to develop behaviors that are intended to force words out or hide and avoid the fact that they do stutter. It also can take a heavy emotional toll.
Yet, people who stutter tend to blame themselves. They grow up continually hearing "Calm down," "Speak slower," and "If you put your mind to it, you won't stutter anymore." People who stutter need to realize that stuttering is not learned; it is the negative emotional reaction to stuttering that is learned.  These reactions are established by negative conditioning and often can lead to depression and often hinder any type of treatment attempted.
 Dennis Drayna, Ph.D., "Telephone Interview with Dr. Drayna, Chief, Section on Systems Biology of Communication Disorders at the National Institutes of Health", November 24, 2009.
 "Stuttering Information", n.d., http://www.westutter.org/stutteringInformation/index.html.
 Barry Guitar, Stuttering: An Integrated Approach to Its Nature and Treatment, 3rd ed. (Lippincott Williams & Wilkins, 2006), 15.
 William D. Parry, Understanding & Controlling Stuttering: A Comprehensive New Approach Based on the Valsalva Hypothesis (William D. Parry, Esq., 2006), 42.
 "Stuttering Foundation of America", n.d., http://www.stutteringhelp.org/Default.aspx?tabid=4.
 "Stuttering Facts and Information | Stammering Information", n.d., http://www.stutteringhelp.org/Default.aspx?tabid=17.
 Robert Logan, The Three Dimensions of Stuttering: Neurology, Behaviour and Emotion, 2nd ed. (London: Whurr Ltd, n.d.), 49, http://www.netlibrary.com/.