In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal.
There are several ADC architectures, in MCU normaly there are SAR (successive approximation register) and sometime SIGMA-DELTA (also known as a delta-sigma ADC).
SAR is very fast
SIGMA-DELTA is slow but is very precise
The stadard ADC in STM32 is SAR at 12bit but there ara also some STM32 that iclude also a SIGMA-DELTA ADC at 16bit.
For reading more, click here.