Description
The ESP8266 ESP-01 is a Wi-Fi module that allows microcontrollers access to a Wi-Fi network. ... Thus, we can give a microcontroller internet access like the Wi-Fi shield does to the Arduino, or we can simply program the ESP8266 to  have both access to a Wi-Fi network and also to act as a microcontroller as well.
 
KEY FEATURES 
32-bit RISC Tensilica Xtensa LX Processor running at 80MHz
1MB Flash Memory
IEEE 802.11 b/g/n WiFi
2 GPIO
3.3V Operation
 
Resources: 
1. Using ESP-01 and Arduino UNO
2. How to Program ESP8266 (ESP-01) Module with Arduino UNO