Vedic Sanskrit
Vedic Sanskrit is the language of the Vedas, a large collection of hymns, incantations (Samhitas), theological discussions, and religio-philosophical discussions (Brahmanas, Upanishads) which are the earliest religious texts of the Hindu religion.
Classical Sanskrit
The Vedic form survived until the middle of the first millennium BC. Around this time, as Sanskrit made the transition from a first language to a second language of religion and learning, the Classical period began.It is a significant form of post-Vedic Sanskrit and is found in the Sanskrit of the Hindu Epics—the Ramayana and Mahabharata. There have been “prakritisms” (borrowings from common speech) in this form of Sanskrit.
Classical Sanskrit became fixed with the grammar of Panini (roughly 500 BC), and remains in use as a learned language until the present day.