Fluency can be thought of simply as a speaker's effortless flow of speech.
Stuttering/Dysfluency is the condition in which the flow of speech is broken by abnormal blocks (no sound), repetitions (st-st-stuttering), or prolongations (ssssstuttering) of sounds and syllables. There may also be unusual facial and body movements associated with the effort to speak.
Cause: We still do not know for a fact what causes stuttering. It may have different causes in different people, or it may only occur when a combination of factors comes together. Possible influences include incoordination of the speech muscles; rate of language development; the way parents and others talk to the child; and other forms of communication and life stress.