Each ESP8266 ESP-01 module comes pre-programmed with an AT command set firmware, meaning, you can simply hook this up to your Arduino device and get about as much WiFi-ability as a WiFi Shield.
The GPIO pins allow Analog and Digital IO, plus PWM, SPI, I2C, etc.
This module has on-board 80 MHz low power 32 bit processor which can be used for custom firmware. This also means that you can host small WebPages without any external controller.
When ESP8266 ESP-01 hosts the application, and when it is the only application processor in the device, it is able to boot up directly from an external flash. It has integrated cache to improve the performance of the system in such applications, and to minimize the memory requirements.
ESP8266 is transforming the world with its low cost and high features which makes it an ideal module for Internet of Things (IOT). It can be used in any application where you need to connect a device to your local network or internet.
These modules include 1MB (8Mbit) of flash memory, twice the size of the older blue colored ESP-01 module
The ESP8266 Serial/UART to WiFi module is a great way to connect your Arduino or other microcontroller projects to a WiFi network. The eight pin header includes two GPIO pins that allow for direct connection of the module to sensors, peripherals, or host controller