Dharana means steadfastness of the mind. This dharana, is to fix or still citta on a particular spot or object.
In dhyana, the mind rests on an object forever. The mind will be totally free from the disturbances of the senses.
In samadhi, citta takes the nature of the object of thought, free from any fancy, imagination. Samadhi is free from Kalpana, fancy.
When there is dharana, dhyana and samadhi, all three on one particular object, it is samyama. Samyama is the technical term of the science of yoga to denote these three.