The ESP-01S, ESP8266 WiFi Module is a self-contained SOC with integrated TCP/IP protocol stack that can give any microcontroller access to your WiFi network. The ESP8266 is capable of either hosting an application or off loading all Wi-Fi networking functions from another application processor. It is very useful for simple IoT.
Each ESP-01S ESP8266 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 offers (and thats just out of the box). The ESP-01S ESP8266 module is an extremely cost-effective board with a huge, and ever-growing, community.
This module has a powerful enough on-board processing and storage capability that allows it to be integrated with the sensors and other application-specific devices through its GPIOs with minimal development up-front and minimal loading during runtime.
Its high degree of on-chip integration allows for minimal external circuitry, including the front-end module, is designed to occupy minimal PCB area. The ESP8266 supports APSD for VoIP applications and Bluetooth co-existence interfaces, it contains a self-calibrated RF allowing it to work under all operating conditions and requires no external RF parts.
ESP8266 WiFi Features:
1. 1MB (8Mbit) Flash size.
2. Very competitive price
3. AP, STA, AP+STA modes available.
4. Very simple AT commands, easy to use.
5. Internal LWIP interface.
6. Four pins interface, VCC-3V3, GND, TXD, and RXD. Two extra GPIOs can use.
7. AT commands firmware. New IC ESP8266ex.