What is the difference between an injected and regular STM32 ADC channel?
You can configure the ADC to read in a sequence of channels in a loop. Those channels are being converted regularly. In injected mode conversion is triggered by an external event or by software. An injected conversion has higher priority in comparison to a "regular" conversion and thus interrupts the regular conversions.

The different ADC-Modes are explained in application note AN3116.