Concentration(dharana) : “When you establish the internal reality of God consciousness in Your soul, that is dharana. This dharana is not only to be established in internal God Consciousness but in all activities of the worldly life. This is true dharana." (Netra Tantra.)
1 The holding of consciousness in the emptiness of the space is concentration (dharana) or the connectedness of the consciousness with the object of contemplation is concentration ...
2 There is that tune in a single experience - act Meditation (dhyana).
3 Only the meditation, which brings the object to light alone, and whereby one is exposed to one's own identity, is meditation (samadhi) - Patanjali, Yogasutras III, A.1
MEDITATION(Skt. dhyana; Pali Jhana) : Spatial concentration of consciousness is Dharana; the from thinking in a directed following tension is Dhyana, as long as it only shines in a meaningful appearance.(Yogasutras - 3 :Vibhuti Pada)
Focus the mind on inner consciousness and then meditate the Ista Devata in the Jivatma. This is the Dhyâna. (Devi Bhagavatam Purana, Buch 7.35)
The Kulanarva Tantra defines 'Dhyana' as : The mastery of suffering of the senses by the mind, the contemplation by the inner being of the chosen Godhead is called dhyana ( translation by Rai).
Blavatsky : Voice of Silence
Meditation is a far-reaching concept, which in practice extends from a quiet relaxation, the rejection of thoughts, the contemplative thinking about special contents, the mental repetition of words (like in the old rosicrucian "Ich Bin" - meditation), the mental repetition of indish Mantras to the inner multiple concentration as for example described here.
Widespread exercises are :
At this the Mantra-Mediation has the special purpose, to create a resonance and vibration which amplifies itself by the constant concentrated repetition and which transforms those regions, which come into contact with this vibration. The deeper the meditation the bigger are the effects of such a vibration.
The concentrated repetition of a mantram creates in the EEG a growing synchronization of the electric brain activity, which amplifies the rhythm of the alpha-waves and causes a mental and physical relexation. 
Prerequisite for such meditation is traditionally the control of the thoughts. This has the simple grounds in the fact, that thoughts disturb the concentration on the Mantram. Otherwise such concentration is a cramped holding tight at the mantram. It should be an "IT - istic" looking recur, corresponding with the state of consciousness of the student. : IT acts in the transcendent areas....
A futher reason of the necessity of the previous control of the thoughts is simply, that storms of thoughts that happen in deep meditation can have later chaotic effects, when the deep mental waves come into realization in life. As soon as such chaotic thoughts have transcendental effects in the Akasha, they can have strong karmic effects. Therefore a deep meditation with storms of thoughts can have very negative effects, even if the student notices that and then returns to the mantram.
Another point is, that the deep meditation about a mental content or a mental theme does not take into account, that mental waves have a quabbalistic aspect, so that such uncontrolled thoughts and badly composed mantrams can often have very negative unconscious effects but must not have . (--> Fr. Bardon : "Key to the True Quabbalah")
Generally has meditation three steps :
1. Preparation : Attainment of a quietness, for example through the concentration on the breath or using Mantrams, which have such effects(in Taoism "S-Ung" (quietness) or "F-ANG" (relaxation) or "So-Ham", or even through observation and rejection of upcoming thoughts. Another method is to read a chapter of a holy book, so that the contents of the momentary thoughts are tuned. A further metod is the visualisation of the universe as filled with consciousness.
2. The Mediation with a certain technique within several steps (until the later Samadhi)
3. The slow return to the normal consciousness without haste.
Here we can find methods of meditation over elementary forms like triangles and Mandalas or Lotus Flowers, of pictures of deities or buddhas or masters, over cosmic mandalas with a great variety of forms and colours.
Such visualisations make use of the lawful contents of colour and its meaning, and of form and force and of the qualities of those forces. (the Feng Shui offers to this one disputed explanation).
Especially in Tantrism (Mandala, Yantra) and even in Buddhism(Vajrayana) we can often find such visual meditations. Generally these techniqs were used by progressed aspirants with trained abilities of visualization and imagination.
The simultaneous use of sound, colour and meaning makes these exercises much more effective, but because they are difficult to practice, the practise of the single aspects will mostly be necessary before that.
Therefore Franz Bardon teaches the use of letters in combination with colour and specific feelings in his book about the Quabbala, and he explains their individual meaning.
For example in hinduism the meaning and the colours and characteristics of a deities were exactly explained by a real guru before initiation.