In August 2014 Espressif (at that time an unknown Chinese company) released the ESP8266. It was meant to be used as a Wi-Fi bridge for other microcontrollers until someone noticed it had way more processing power and memory than those other microcontrollers.
Later that year in November, the SDK went public and a hacker (Ivan Grokhotkov, now working for Espressif) released a project to support the ESP8266 from within the Arduino IDE. And as they say in the books the rest is history.
Today Espressif makes 2 of the most famous silicon chips used in IOT eco-system, namely ESP32 and ESP8266. Here you can find various introduction, how-to and security related articles for these chips.
1. What are ESP32, ESP8266 Modules and Development Board
2. AT Commands for ESP8266: Where and How to use them
3. How ESP8266-01 works : Pinout and Connecting to Arduino
4. How to flash ESP8266-01 – Default/Custom Firmware Upgrade
5. How to make a HTTPS (secure) connection using ESP8266