Yoga can mean many things to many people as it is an individual / spiritual practice to create self awareness.
In sanskrit yoga derives from the word YUJ which means to join or unite. Yoga is union of the individual consciousness (soul) with the universal consciousness (spirit) we arrive at this meeting point, this union, through asana (yoga postures) pranayama (breathing techniques) and meditation.
Asana is designed to strengthen the physical body & nervous system – strengthening the spine and all internal organs and thus enhancing their function.
Pranayama is intended to regulate ones breathing, when ones breaths are deep and full the mind is in a relaxed and calm state.
Meditation is a skill which once practiced daily, like exercising a muscle, can be an effective form or stress reduction and has the potential to improve your quality of life by calming the minds chatter.