In these dialogues, Socrates is the central character, and is believed to be expressing his own views. These are the only remaining record of Socrates' teachings; hence these are known as the Socratic dialogues.
In these dialogues, Plato begins expressing his own views, in the guise of Socrates. The Symposium and Republic are the most important works in this period.
The later dialogues are deeper developments of the philosophy expressed in the earlier ones; these are the most difficult of Plato's works.