The idea of segments

• your tongue tip raises from a relatively neutral position till it touches your alveolar ridge and cuts off airflow through the mouth
• your tongue body starts in a high position and drops to a relatively neutral position
• your lips start out rounded and become less rounded
• your soft palate (velum) starts out raised, blocking off airflow through the nose, but becomes lower and lower, allowing more and more air out through the nose
• your vocal cords are vibrating

The parts of the vocal tract are (mostly) independent of each other. Several parts are changing, but they aren't necessarily changing all at the same time. For example, for most English speakers saying [un], the soft palate lowers and lets air out through the nose long before the tongue tip raises and blocks off airflow through the mouth. The whole set of movements by four parts during the word spoon might look like:

(1)

It certainly doesn't look like:

(2)

On a practical level, it's almost impossible to start work using the first kind of picture, even if it is more realistic. To begin studying phonetics, we have to idealize the real world. We have to act as if speech events look like the second picture rather than the first. We can divide the idealized picture up into segments which have well defined properties. Figure (2) has four segments:

1. a segment with unrounded lips, the tongue tip near but not touching the alveolar ridge, neutral tongue body; raised soft palate, and spread vocal cords -- we can write this as [s];
2. a segment with unrounded lips, neutral tongue tip, neutral tongue body, raised soft palate, and spread vocal cords -- we can write this as [p];
3. a segment with rounded lips, neutral tongue tip, high tongue body, raised soft palate, and vibrating vocal cords -- we can write this as [u]; and
4. a segment with unrounded lips, the tongue tip against the alveolar ridge, neutral tongue body, lowered soft palate, and vibrating vocal cords -- we can write this as [n].

When phoneticians and phonologists talk about a sound, they are usually referring to one of these idealized segments.

Ultimately, a theory of phonetics and phonology has to explain realistic pictures like (1) rather than simplified pictures like (2). But for many practical purposes, it's good enough if we use the idealization that any stream of speech can be broken up into a series of segments and each segment given a phonetic symbol.