Stuttering is a speech disorder where pronunciation of the (usually) first letter or syllable of a word is repeated involuntarily. For example, stuttering on the word "table" would become either "t-t-t-t-...t-table" or "ta-ta-ta-ta-...-ta-table". Stuttering is an involuntary process which hinders normal communication. The person stuttering cannot himself stop the process, but anxiousness or nervousness often escalates the problem.
Usually, stuttering appears slightly before adolescence, and sometimes it disappears after puberty. However, if it continues to a mature age, it usually stays forever. Stuttering can be treated through therapy, but the results vary. Most commonly, the problems can be decreased, but there is no objective way to measure the results.
Stuttering sometimes develops into blocking, that is, even the first letter or syllable becomes impossible to pronounce, i.e. blocked. A natural explanation for this is that while trying to avoid stuttering or in fear of stuttering, the person cannot utter the word at all. This too is an unvoluntary action even though the development from stuttering to blocks can be partly intentional.
In the past, this disorder was often attributed to mental problems, but perhaps the most prominent current view is that the reason has something to do with neural synchronisation problems between the two halves of the brain. The disorder can be compared with dyslexia which is another disorder affecting communication.
Famous people who stutter:
As a practical note for non-stuttering people; Most people stuttering would wish that the problem be ignored in normal situations. However, if stuttering causes a problem for the listener, then you should not be afraid to talk about it.