Both Astra and Sastra are terms used to refer to weapons in Hindu Puranas, epics and other texts. But there is a difference between them.
Astra refers to weapons that are associated with mantra or chant. Thus even a blade of grass can become an astra when it is infused with the power of mantra. Most of the arrows used in the Mahabharata and Ramayana fall in the category of astra.