What is fluency?
Fluency is the flow of speech. Fluent speech is smooth, forward-moving, unhesitant and effortless speech. A "dysfluency" is any break in fluent speech. Everyone has dysfluencies from time to time. "Stuttering" is speech that has more dysfluencies than is considered average.
Everyone has dysfluencies in their speech. The average person will have between 7-10% of their speech dysfluent. These dysfluencies are usually word or phrase repetitions, fillers (um, ah) or interjections. When a speaker experiences dysfluencies at a rate greater than 10% they may be stuttering. Stuttering is often accompanied by tension and anxiety. The types of dysfluencies in stuttering may also be different. Sound or syllable repetitions, silent "blocks", prolongations (unnatural stretching out of a sound) and secondary characteristics such as facial grimaces or tics can be present.
Many children go through a period of normal dysfluency between the ages of 2 and 5 years of age. The frequency of dysfluency can be greater than 10%. The dysfluencies are usually whole word or phrase repetitions and interjections. The word is repeated just once or twice and is repeated easily. The child does not demonstrate any tension in their speech and is often unaware of their difficulty. It has been suggested that the cause of this nonfluency may be a combination of increases in language development, development of speech motor control, or environmental stresses that can occur in typical busy families. Some children "outgrow" these dysfluencies, others do not.
Some famous people that stutter are:
Ed Sheeran, Bruce Willis, Charlie Sheen, Elvis Presley & Michelle Williams (from Destiny's Child)
This chart looks at the various risk factors for stuttering.
This is the Stuttering Foundation's main website with great information for children and adults who stutter.