Chapter 1. Getting Started with STM32

According to ST's official website http://www.st.com/en/microcontrollers/stm32-32-bit-arm-cortex-mcus.html,

The STM32 family of 32-bit Flash microcontrollers based on the Arm® Cortex®-M processor is designed to offer new degrees of freedom to MCU users. It offers products combining very high performance, real-time capabilities, digital signal processing, and low-power and low-voltage operation, and connectivity, while maintaining full integration and ease of development.

STM32's Ecosystem can be summarized as:

STM32 32-bit ARM Cortex MCUs and STM32 Ecosystem