“If you focus the rays of the sun through a lens, they can burn cotton or a piece of paper; but, the scattered rays cannot do this act. If you want to talk to a man at a distance, you make a funnel of your hand and speak. The sound-waves are collected at one point and then directed towards the man. He can hear your speech very clearly. The water is converted into steam and the steam is concentrated at a point. The railway engine moves. All these are instances of concentrated waves. Even so, if you collect the dissipated rays of the mind and focus them at a point, you will have wonderful concentration. The concentrated mind will serve as a potent searchlight to find out the treasures of the soul and attain the supreme wealth of Atman (the Self), eternal bliss, immortality and perennial joy.
Real Raja Yoga starts from concentration. Concentration merges in meditation. Concentration is a portion of meditation.
Meditation follows concentration. Samadhi (superconscious state) follows meditation. The Jivanmukti (liberated being) state follows the attainment of Nirvikalpa Samadhi which is free from all thoughts of duality. Jivanmukti leads to emancipation from the wheel of birth and death. Therefore, concentration is the first and foremost thing a Sadhaka or aspirant should acquire in the spiritual path.
Concentration or Dharana is centering the mind on one single thought. Concentration is a great necessity for all aspirants. When there is deep concentration, there is no consciousness of the body and surroundings. This is concentration or one-pointedness of mind. The mind is fixed firmly on one thing. You must have such a deep concentration when you think of God or the Atman.”
Swami Sivananda
Concentration
http://www.dlshq.org/teachings/concentration.htm