Arduino is a convenient, flexible, and easy-to-use open source electronic prototyping platform. Contains hardware (various models of Arduino boards) and software (Arduino IDE). Developed by a European development team in winter 2005. Its members include Massimo Banzi, David Cuartielles, Tom Igoe, Gianluca Martino, David Mellis and Nicholas Zambetti.
It is built on the open source simple I/O interface version and has a Processing/Wiring development environment similar to Java and C. It mainly consists of two main parts: the hardware part is the Arduino circuit board that can be used for circuit connection; the other is the Arduino IDE, the program development environment in your computer. You just write the program code in the IDE, upload the program to the Arduino board, and the program will tell the Arduino board what to do. (Source Baidu Encyclopedia)
In layman's terms: arduina is a hardware open source development board. The processor of this board is Atmel's AVR series, which is different from the ARM architecture described below.
And these are open source and can be downloaded from their official website
The ARM processor was the first RISC microprocessor designed by Acorn Ltd. for the low-budget market. The full name is Acorn RISC Machine. The ARM processor itself is a 32-bit design, but is also equipped with a 16-bit instruction set, which generally saves up to 35% over the equivalent 32-bit code, but retains all the advantages of a 32-bit system.
In layman's terms: ARM refers to a processor architecture, a processor core, and is also the name of ARM company and the name of ARM technology.
As the processor architecture is different from X86 PowerPC, etc., some of our common mobile phone chips, and some control chips, many use ARM architecture, such as Nvidia's Tegra series in the mobile phone field, Samsung Electronics' Orion series, in the field of industrial control The common STMicroelectronics STM32 series are all ARM architectures.
Raspberry Pi (Chinese name "Raspberry Pi", abbreviated as RPi, (or RasPi / RPI) is designed for students' computer programming education. It is only a credit card-sized card computer, and its system is based on Linux.  With Windows With the release of 10 IoT, we will also be able to use the Raspberry Pi running Windows. Since its inception, it has been sought after by many computer enthusiasts and makers. , but the inner "heart" is very powerful, and it has all functions such as video and audio.
Raspberry Pi refers to a small computer system with open source hardware and is now very popular. Its essence is also a development board. Its chip is Broadcom's BCM2835 chip. This chip uses the above-mentioned ARM architecture. Most of the things in your computer case are integrated into a microcomputer on a circuit board. There is no substantial difference between the monitor, mouse, keyboard, etc. connected to your computer. The operating system is different. This is a linux-based system.
Microcontrollers (Microcontrollers) is an integrated circuit chip, which uses the ultra-large-scale integrated circuit technology to integrate the central processing unit CPU, random access memory RAM, read-only memory ROM, various I/O ports and interrupt systems, timers with data processing capabilities. /Counter and other functions (may also include display drive circuits, pulse width modulation circuits, analog multiplexers, A/D converters and other circuits) are integrated into a small and complete microcomputer system on a silicon chip. It is widely used in the field of control. From the 1980s, from the 4-bit and 8-bit single-chip microcomputers at that time, it has developed into the current 300M high-speed single-chip microcomputer.
Single chip microcomputer is called MCU microcontroller abroad
It is to integrate CPU, RAM, ROM, etc. into a single chip to form a single-chip microcomputer.
Arduino: Simple and complete resource-rich open source microcontroller application development board
ARM: High-performance microcontroller
Raspberry Pi: The Micro Card Computer
Microcontroller: A single-chip computer