Speech starts out through muscle movements that are in sync with one another. There are different parts, such as:
The brain is the starting point where it controls and coordinates these particular muscle movements. The muscle movements are closely watched by hearing and touching.
Prior to a person speaking, a person takes a breath and their vocal cords will come together. The sound that comes from the voice travels through the throat and goes through the mouth when people are speaking, for the most part.
With nasal sounds, it travels through the nose. The jaw, tongue, palate and lips move in specific ways to change sounds in order to create speech sounds. With stuttering, it doesn't have to do with the creation of speech sounds, per se.
Nor it is associated with placing thoughts with words. Stuttering does not have any affect on how intelligent a person is or lack thereof. People that stutter may be some of the smartest people to ever walk the face of the earth.