Request for Quotation (RfQ) is used when you plan to purchase some products and you would like to receive a quote for those products. In Odoo, the Request for Quotation is used to send your list of desired products to your supplier. Once your supplier has answered your request, you can choose to go ahead with the offer and purchase or to turn down the offer.
A Purchase Tender (PT), also known as Call for Bids, is used to drive competition between several suppliers in order to get the best offer for a list of products. In comparison to the RfQ, a Purchase Tender is sent to multiple suppliers, stating each are competing with one another, and that the best offer will win. The main interest is that it usually leads to better offers.
The Purchase Order (PO) is the actual order that you place to the supplier that you chose, either through a RfQ, a Purchase Tender, or simply when you already know which supplier to order from.