UPI is a single-window mobile payment system launched by NPCI that allows users to instantly transfer money between bank accounts using only a virtual payment address. It eliminates the need for users to enter sensitive bank details with each transaction. To use UPI, a user simply downloads a participating bank's app, registers with their virtual ID like phone number, and can then send and receive money instantly. For online purchases, a user only needs to provide their virtual ID to the merchant and authenticate the transaction via their bank app. UPI aims to facilitate a digital and cashless society in India by making payments simple, secure and convenient through a single mobile interface.