ESP-WROOM-02 is a low-power 32-bit MCU Wi-Fi module, based on the ESP8266 chip. TCP/IP network stacks, 10-bit ADC, and HSPI/UART/PWM/I2C/I2S interfaces are all embedded in this module.
ESP-WROOM-S2 uses a 2 MB SPI flash connected to HSPI, working as the SDIO/SPI slave with the SPI speed of up to 8 Mbps.