This is WiFi serial transceiver module, based on ESP8266 SoC.  The SOC has Integrated TCP/IP protocol stack.

This is the latest upgraded version of ESP-01 (ESP-01E) with 1M Flash

ESP8266 is a highly integrated chip designed for the needs of a new connected world. It offers a complete and self-contained Wi-Fi networking solution, allowing it to either host the application or to offload all Wi-Fi networking functions from another application processor.
 
ESP8266 has powerful on-board processing and storage capabilities that allow 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, and the entire solution, including front-end module, is designed to occupy minimal PCB area.
 
Corresponding Interface:

•     SDIO 2.0, SPI, UART
•     32-pin QFN package
•     Integrated RF switch, balun, 24dBm PA, DCXO, and PMU
•     Integrated RISC processor, on-chip memory and external memory interfaces
•     Integrated MAC/baseband processors
•     Quality of Service management
•     I2S interface for high fidelity audio applications
•     On-chip low-dropout linear regulators for all internal supplies
•     Proprietary spurious-free clock generation architecture
•     Integrated WEP, TKIP, AES, and WAPI engines

Specification:

•     Firmware : [Vendor:www.ai-thinker.com Version:0.9.2.4]
•     802.11 b/g/n
•     Wi-Fi Direct (P2P), soft-AP
•     Integrated TCP/IP protocol stack
•     Integrated TR switch, balun, LNA, power amplifier and matching network
•     Integrated PLLs, regulators, DCXO and power management units
•     +19.5dBm output power in 802.11b mode
•     Power down leakage current of <10uA
•     Integrated low power 32-bit CPU could be used as application processor
•     SDIO 1.1/2.0, SPI, UART
•     STBC, 1×1 MIMO, 2×1 MIMO
•     A-MPDU & A-MSDU aggregation & 0.4ms guard interval
•     Wake up and transmit packets in < 2ms
•     Standby power consumption of < 1.0mW (DTIM3)

RF specifications

The following data is at room temperature, the voltage of 3.3V and 1.1V, respectively, when measured

Note: For normal operation, Vcc and CH_PD (chip enable) pins are tied to 3.3V with GND to 0 V, and TX and RX pins connected to your serial interface of choice (Arduino, FTDI USB interface, etc.). As the module operates at 3.3v, if your serial interface operates at 5v, you will need a level converter in the module’s RX line to prevent over-voltage. A simple resistor divider (1kΩ and 2.2kΩ) or resistor and 3.3v zenner will do the trick. For better performance, use a FET level shifter in both RX and TX lines.

For detailed information on this module visit here or here

Another Interesting Blog is here and here

An interesting video is here and here and another one on how to make a breadboard adapter for ESP-01

Software: 

• ESP8266 IoT SDK
• Datasheet

Sample Codes: 

1. ESP8266_HelloWorld.ino
2. ESP8266_LM35_Temperature.ino
3. ESP8266_Test.ino

Sample AT Command Test: