Electronic Controller Unit (ECU) Vs. MCU
A Microcontroller is an integrated circuit (IC) that is usually a system on chip (SOC) meaning its processor core, memory and IO are all inside one discrete package. Microcontrollers are generic so to use them for specific functions they require supporting hardware and software.

In comparison an Electronic Control Unit (ECU) is a general term that abstracts from the hardware beneath it. 

A Microcontroller can be used to make an ECU by integrating it on a circuit board with appropriate physical connectors, custom software, power circuitry and mechanical packaging. However an ECU could instead use an FPGA, mini-PC or purely analogue hardware