STM32 bus

AHB , is an abbreviation for Advanced High performance Bus is translated as advanced high-performance bus, this is a "system bus."
AHB is mainly used for a high-performance connection between a module (e.g., CPU, DMA and DSP, etc.). AHB system by the master module, slave module, and the composition of base structure (Infrastructure) 3 portion, the entire transmission on the AHB bus by the master module issues, is responsible for the response from the module.

APB , is an acronym for Advanced Peripheral Bus, which is a peripheral bus.
APB mainly used to connect peripheral devices between the low-bandwidth, e.g. UART, 1284, etc., it is not AHB bus architecture supports multiple master module, the main module is the only APB inside the APB bridge.