Yoga sutra iii.4 “trayam ekatra samyamah”

“These three together – dharana, dhyana and samadhi – constitute integration or samyama.”
Dharana is to focus one’s attention on a point whether external, a spot on the wall, or internal the word ohm and to concentrate.  When the point of attention ceases to exist, when nothing else exist for the mind this state has become dhyana, or meditation.
Dhyana or mediation occurs when awareness of physical self, our body or the world around us, is fully absorbed within our consciousness.  When we no longer feel our feet, hands, etc., our minds are silent and all we feel is the energy of life.  This may manifest itself as calmness, a low humming sound, a vibration, maybe even a tingling feeling throughout the body.  Mediation is sometimes expressed as the merging of our true selves with the universe.  It is during meditation that samadhi is experienced.
Samadhi is known as spiritual absorption, union with, tranquility.  Samadhi is complete absorption and is experienced through dhyana or mediation.
Samyama is the trinity of the three, dharana, dhyana, samadhi.  Samyama is a process, a cycle where all three parts of this trinity exist and become one experience, samyama.  The progression begins with dharana and this concentration on something outside of ourselves brings us into ourselves and becomes concentration on ourselves, dhyana.  At some point we completely lose ourselves within dhyana and this is when we experience samadhi.  At this point samyama or integration is occurring.