Laver (1994) distinguishes between three phases of a segment: the onset, medial, and offset phases. In an alveolar [t], for example:
In general, most segments are defined by the position of the articulators during the medial phase. This is the phase of consonants when the constriction in the vocal tract is at its narrowest. The onset and offset phases are usually nothing more than the movements that are required to get the vocal tract to and from that most narrow state. This isn't to say they're unimportant. Acoustically, the medial phases of [p], [t], and [k] are complete silence -- all the information that a listener can use to tell the sounds apart comes during the onset and offset phases.
Phases can be used to describe almost any kind of segment, not just stops. For vowels, the medial phase is the time during which the tongue body is at the definining position for that vowel (high, low, front, back, etc.). This isn't necessarily when the vocal tract reaches its narrowest point, as it is for stops and fricatives. It can be, as seen in the following diagram for tongue body position during the [u] of [tut]:
But the medial phase for vowels might also be the period where the vocal tract is at its widest , as in the [u] of [kuk]:
(The onset-medial-offset method of describing segments doesn't work particularly when when applied to diphthongs.)
While most segments are defined by what happens during their medial phase, some kinds of segments have interesting properties that involve the onset or offset phases. Aspiration, for example, can be seen as a property which holds during the offset or release phase of a plosive. Affrication can also be seen as a modification of the offset phase of a plosive.