UPI stands for Unified Payments Interface. It is a system that brings together multiple bank accounts (of participating banks) into a single mobile application, merging different banking features and merchant payments under a single roof. To explain in lucid language, if you have bank accounts in different banks registered with the same phone number, you can access all of your accounts through a single app. The app brings all your bank accounts under a single UPI ID through which you can transfer money, pay to merchants, and access several other banking facilities. It is entirely a mobile-based interface, wherein, most transactions happen on the mobile through a unique ID known as VPA (Virtual Payment Address). Bank account details, account number, IFSC codes, etc. are not required in UPI transactions.
What is UPI? Watch here
What does UPI Support? Watch here
Which banks are members of UPI? Watch here
Axis Bank UPI video Watch here
PNB UPI Set-up Video Watch here
UPI FAQs Watch here