Dhyana, the seventh limb of Ashtanga Yoga, means worship, or profound and abstract religious meditation. It is perfect contemplation. It involves concentration upon a point of focus with the intention of knowing the truth about it.
During dhyana, combining clear insights into distinctions between objects and the subtle layers surrounding intuition further unifies the consciousness. We learn to differentiate between the mind of the perceiver, the means of perception, and the objects perceived—between words, their meanings and ideas, and even between all the levels of natural evolution. We realize that these are all fused in an undifferentiated continuum. One must apprehend both subject and object clearly in order to perceive their similarities. Thus dhyana is apprehension of real identity among apparent differences.
During dharana, the mind becomes unidirectional, while during dhyana, it becomes ostensibly identified and engaged with the object of focus or attention. That is why, dharana must precede dhyana, since the mind needs focusing on a particular object before a connection can be made. If dharana is the contact, then dhyana is the connection.
Obviously, to focus the attention to one point will not result in insight or realization. One must identify and become "one with" the object of contemplation, in order to know for certain the truth about it. In dharana the consciousness of the practitioner is fixed on one subject, but in dhyana it is in one flow.