Dharana means to fix the mind on a pellicular object / subject without fluctuating the attention or mind. Dhyana: This requires total meditation on the object of concentration. One has clear knowledge of the object or subject by this process. In the beginning, the aspirant knows only the forms of the object and gradually realizes the whole of it. Or oneness with the object or subject is dhyana (tatprati ekatanata dhyanam.) samadhi: This is the last and most important stage of Yoga.
The above seven stages are only preparatory steps to reach this stage. When the process of concentration and this object become one, without any difference between the subject and object, this oneness is known as samadhi or in the other words there is on absence of form (rupa); only artha or meaning will be present in samadhi (tadevarthamatranirvasasvarupah sunyamivasamadhi)