PETROFF DEFENCE.... WHAT IS IT ALL ABOUT?

The Petroff: Move by Move
https://www.newinchess.com/media/wysiwyg/product_pdf/7530.pdf

I think you need to be more specific in your question. Petrov's Defense is the *only* solid reply to 1. e4 e4 2. Nf3 that counterattacks instead of defends. That makes it pretty cool, right from the start.