The Unified Payments Interface (UPI) provides a single interface for online payments across all NPCI systems using standard APIs. It aims to simplify payments and improve customer experience through interoperability. UPI allows for instant payments through a single click using two-factor authentication on mobile. It also enables use of virtual payment addresses instead of sharing sensitive bank details. UPI transactions use a central repository to route payments between participating banks in real-time, with strong security features like encryption and digital signatures.