What are the differences between ARM Cortex-M and Cortex-A series?

Cortex-M is designed for microcontroller applications: lower power, smaller area, deterministic interrupt response, no memory management unit (simpler programming model), and Thumb-2 instruction set. Runs bare-metal or RTOS. Cortex-A is for application processors: higher performance, full MMU for virtual memory and OS support (Linux, Android), out-of-order execution, cache hierarchy, and full ARM instruction set with NEON SIMD. Cortex-M suits sensors, motor control, IoT; Cortex-A suits smartphones, tablets, embedded Linux systems.