Fare collection and tolling is the set of systems that charge people as they travel: a card tapped at a metro gate, a ticket bought for a city bus, and a FASTag read at a toll plaza. Each fare or toll is worked out and charged by a back office. A back office is the computer system, and the team that runs it, that does the accounts behind the gate or the lane.
A metro or bus system needs gates and readers, and a back office. The back office works out each fare, keeps the accounts, and shares the money among operators when one trip uses more than one. This whole system is called automatic fare collection. In India, a RuPay card called the National Common Mobility Card (NCMC) is meant to pay for metros, buses and tolls with a single card.
Bus riders also use an app. Bus tracking and digital tickets shows where the bus is and sells the ticket. On highways, electronic tolling reads a tag or a number plate and charges the owner's account. Both rely on reading a vehicle or card automatically, through RFID for tags and automatic number plate recognition for cameras.
Imagine one day in Chennai. In the morning you check the bus app to see when your bus arrives and buy the ticket. At noon you tap a card at a metro gate. In the evening your car passes a toll plaza on the highway out of the city, and the toll is charged to your FASTag account. Three different systems charged you, and each has its own back office. None of them knows what the others charged.
The buyers of these systems are city transport authorities, metro and bus companies and toll-road operators. They buy through government tenders. The systems run for decades, because replacing gates and readers is expensive and disruptive, and because a fare system that stops working stops a whole city's travel.