ESP8266 is a SOC, System on Chip, with Wifi built-in. This chip have CPU at 80Mhz and have the option to operate to 160Mhz. To have a compare, Arduino Uno operates at 16Mhz, it is a stark diference.
ESP8266 have RAM with 32kBytes for instructions and 96kBytes for data. It also have ROM with 64kBytes and Flash memory with 512kBytes. The small ESP8266 have GPIO pins, I2C, SPI, UART, analogic input, PWM output and internal temperature sensor.
