What is Purchase Requisition How to Create PO from any Purchase Requisition in SAP I Adopt PR in PO 

What is Purchase Requisition, How to Create PO from any Purchase Requisition in SAP, I Adopt PR in PO,
How to Adopt PR in creation of PO, I
ME51N- Creation of PR,
ME52N- Any changes in PR,
ME53N-Display PR,
PR,
ME54N- Release or Cancel Release pR,
ME21N-Adopt PR in PO,
ME22N- Change PO,
ME23N-Display PO,