ESP8266

The ESP8266 is a low-cost Wi-Fi microchip, with built-in TCP/IP networking software, and microcontroller capability, produced by Espressif Systems in Shanghai, China.
The chip was popularized in the English-speaking maker community in August 2014 via the ESP-01 module, made by a third-party manufacturer Ai-Thinker. This small module allows microcontrollers to connect to a Wi-Fi network and make simple TCP/IP connections using Hayes-style commands. However, at first, there was almost no English-language documentation on the chip and the commands it accepted. The very low price and the fact that there were very few external components on the module, which suggested that it could eventually be very inexpensive in volume, attracted many hackers to explore the module, the chip, and the software on it, as well as to translate the Chinese documentation.
The ESP8285 is a similar chip with a built-in 1 MiB flash memory, allowing the design of single-chip devices capable of connecting via Wi-Fi.
These microcontroller chips have been succeeded by the ESP32 family of devices.

Features

Processor: L106 32-bit RISC microprocessor core based on the Tensilica Diamond Standard 106Micro running at 80 or 160 MHz
Memory of 160 Kbytes of RAM which are segmented into:
* 32 KB instruction RAM
* 32 KB instruction cache RAM
* 96 KB of dRAM which are segmented into 80 KB dRAM for SDK and heap memory, and 16 KB for ROM
External QSPI flash: up to 16 MiB is supported
IEEE 802.11 b/g/n Wi-Fi
* Integrated TR switch, balun, LNA, power amplifier and matching network
* WEP or WPA/WPA2 authentication, or open networks
17 GPIO pins
Serial Peripheral Interface Bus
I²C
I²S interfaces with DMA
UART on dedicated pins, plus a transmit-only UART can be enabled on GPIO2
10-bit ADC
Pinout of ESP-01

The pinout is as follows for the common ESP-01 module:

GND, Ground
GPIO 2, General-purpose input/output No. 2
GPIO 0, General-purpose input/output No. 0
RX, Receive data in, also GPIO3
VCC, Voltage
RST, Reset
CH_PD, Chip power-down
TX, Transmit data out, also GPIO1
SDKs

In October 2014, Espressif Systems released a software development kit for programming the chip directly, which removed the need for a separate microcontroller. Since then, there have been many official SDK releases from Espressif; Espressif maintains two versions of the SDK — one that is based on FreeRTOS and the other based on callbacks.
An alternative to Espressif's official SDK is the open-source ESP-Open-SDK that is based on the GNU Compiler Collection toolchain, maintained by Max Filippov. Another alternative is the "Unofficial Development Kit" by Mikhail Grigorev.
Other SDKs, mostly open-source, include:

Arduino — A C++-based firmware. With this core, the ESP8266 CPU and its Wi-Fi components can be programmed like any other Arduino device. .
— An open-source BASIC-like interpreter specifically tailored for the Internet of Things. Self-hosting browser-based development environment.
ESP Easy — Developed by home automation enthusiasts.
— ESPHome is a system to control your ESP8266/ESP32 by simple yet powerful configuration files and control them remotely through home automation systems.
- open-source firmware, for home automation.
— Open-source FreeRTOS-based ESP8266 software framework.
— Free and open integrated SDK for ESP8266/ESP8285 chips.
Espruino — An actively maintained JavaScript SDK and firmware, closely emulating Node.js. Supports a few MCUs, including the ESP8266.
— Open-source ESP8285/ESP8266 firmware.
— Port of Jones Forth to the ESP8266 microcontroller.
— A port of MicroPython to the ESP8266 platform.
— includes JavaScript language and library support for the ESP8266
Mongoose OS — An open-source operating system for connected products. Supports ESP8266 and ESP32. Develop in C or JavaScript.
NodeMCU — A Lua-based firmware.
— A cross-platform IDE and unified debugger, which sits on top of Arduino code and libraries.
— Forth-inspired programming language for the ESP8266.
— An actively developed asynchronous C/C++ framework with superb performance and multiple network features.
— A version of the Lisp programming language specifically designed to run on processors with a limited amount of RAM.
— A subset of Microsoft's widely-used Visual Basic 6, which has been adapted as a control language for the ZX microcontroller family and the ESP8266.
Zerynth — IoT framework for programming ESP8266 and other microcontrollers in Python.
IOTBAH - is An operating system for Espressif ESP8266
Webserver, easy IoT solution.
Espressif modules

This is the series of ESP8266-based modules made by Espressif:

Name	Active pins	Pitch	Form factor	LEDs	Antenna	Shielded	Dimensions	Notes
	18	1.5 mm	2×9 castellated		PCB trace		18 × 20	FCC ID 2AC7Z-ESPWROOM02.
	18	1.5 mm	2×9 castellated		PCB trace		18 × 20	FCC ID 2AC7Z-ESPWROOM02D. Revision of ESP-WROOM-02 compatible with both 150-mil and 208-mil flash memory chips.
	18	1.5 mm	2×9 castellated		U.FL socket		18 × 20	Differs from ESP-WROOM-02D in that includes an U.FL compatible antenna socket connector.
	20	1.5 mm	2×10 castellated		PCB trace		16 × 23	FCC ID 2AC7Z-ESPWROOMS2.

In the table above, "Active pins" include the GPIO and ADC pins with which external devices can be attached to the ESP8266 MCU. The "Pitch" is the space between pins on the ESP8266 module, which is important to know if the device will be used on a breadboard. The "Form factor" also describes the module packaging as "2 × 9 DIL", meaning two rows of 9 pins arranged "Dual In Line", like the pins of DIP ICs. Many ESP-xx modules include a small onboard LED which can be programmed to blink and thereby indicate activity. There are several antenna options for ESP-xx boards including a trace antenna, an onboard ceramic antenna, and an external connector that allows an external Wi-Fi antenna to be attached. Since Wi-Fi communications generate a lot of RFI, governmental bodies like the FCC like shielded electronics to minimize interference with other devices. Some of the ESP-xx modules come housed within a metal box with an FCC seal of approval stamped on it. First and second world markets will likely demand FCC approval and shielded Wi-Fi devices.

Ai-Thinker modules

This is the first series of modules made with the ESP8266 by the third-party manufacturer Ai-Thinker and remains the most widely available. They are collectively referred to as "ESP-xx modules". To form a workable development system, they require additional components, especially a serial TTL-to-USB adapter and an external 3.3 volt power supply. Novice ESP8266 developers are encouraged to consider larger ESP8266 Wi-Fi development boards like the NodeMCU which includes the USB-to-UART bridge and a Micro-USB connector coupled with a 3.3 volt power regulator already built into the board. When project development is complete, those components are not needed and these cheaper ESP-xx modules are a lower power, smaller footprint option for production runs.
In the Notes column, Flash memory sizes apply to the given module and all those below it in the table. Exceptions which apply to a single module are shown in.

Name	Active pins	Pitch	Form factor	LEDs	Antenna	Shielded	Dimensions	Notes
ESP-01	6	0.1 in	2×4 DIL		PCB trace		14.3 × 24.8	512 KiB Flash and blue PCB from a generic manufacturer. 1 MiB Flash, AI-Cloud and black PCB from AI-Thinker.
ESP-01S	6	0.1 in	2×4 DIL		PCB trace		14.4 × 24.7	1 MiB Flash
	16	1.6 mm	2×9 edge connector		PCB trace		18.0 × 18.0	Uses ESP8285.
ESP-02	6	0.1 in	2×4 castellated		U.FL socket		14.2 × 14.2
ESP-03	10	2 mm	2×7 castellated		Ceramic		17.3 × 12.1
ESP-04	10	2 mm	2×4 castellated		-		14.7 × 12.1
ESP-05	3	0.1 in	1×5 SIL		U.FL socket		14.2 × 14.2
ESP-06	11	various	4×3 dice		-		14.2 × 14.7	Not FCC approved.
ESP-07	14	2 mm	2×8 pinhole		Ceramic + U.FL socket		20.0 × 16.0	Not FCC approved.
ESP-07S	14	2 mm	2×8 pinhole		U.FL socket		17.0 × 16.0	FCC and CE approved.
ESP-08	10	2 mm	2×7 castellated		-		17.0 × 16.0	Not FCC approved.
ESP-09	10	various	4×3 dice		-		10.0 × 10.0
ESP-10	3	2 mm	1×5 castellated		-		14.2 × 10.0
ESP-11	6	1.27 mm	1×8 pinhole		Ceramic		17.3 × 12.1
ESP-12	14	2 mm	2×8 castellated		PCB trace		24.0 × 16.0	FCC and CE approved.
ESP-12E	20	2 mm	2×8 castellated		PCB trace		24.0 × 16.0	4 MiB flash.
ESP-12F	20	2 mm	2×8 castellated		PCB trace		24.0 × 16.0	FCC and CE approved. Improved antenna performance.
ESP-12S	14	2 mm	2×8 castellated		PCB trace		24.0 × 16.0	FCC approved.
ESP-13	16	1.5 mm	2×9 castellated		PCB trace		W18.0 × L20.0	Marked as "FCC". Shielded module is placed sideways, as compared to the ESP-12 modules.
ESP-14	22	2 mm	2×8 castellated +6		PCB trace		24.3 × 16.2	Mostly advertised with "AI Cloud Inside".

ESP8266

Features

Pinout of ESP-01

SDKs

Espressif modules

Ai-Thinker modules