This WiFi module is based on ESP8266 Integrated Chip from Espressif, offering a complete WiFi networking solution. This low cost WiFi module finds application with various IoT devices. ESP-01 is very compact (25mm x 15mm) with convenient pin outs (standard 2x4 pin header). These boards like the other ESP8266 series boards run on 3.3V (and application of 5V may damage the module. Level shifter would be required to use this module with 5V devices/boards) and like some of the others of ESP8266 series devices have an on-board antenna..
It can be used either to host the applications or to offload all WiFi networking functions from another application processor. Rich interface (GPIO, SPI, SDIO, I2C) options gives this boards potential to be a standalone MCU with WiFi capability for wide range of applications.