To begin, I want to give you a very brief overview of the components in your ESP8266 microcontroller. Below you can see a diagram that should more or less match your board (though maybe not exactly, since this board is open source and there are a lot of different manufacturers):

The ESP8266 microcontroller chip is the bigger silver colored box. Above it you can see a zig-zag gold line. That is the Wi-Fi antenna. You can see 15 pins on each side, each with a label that should be printed on the board, with names such as D0, D1, 3V3, GND, etc. To make matters confusing, the diagram shows that all the pins have other labels shown on the outside, usually associated with their function.

There are three pins labeled 3V3 or 3.3V, which provide electric current. The name derives from the voltage, which is 3.3 volts. These pins can be used to deliver input current to other components that are part of your circuit. There are also four GND or "ground" pins. These provide a ground connection, which is required to close the circuit when you connect a component. Electric current always flows towards the ground, so when you connect a device to the microcontroller you will be making two actual connections. A connection from a 3.3V pin will deliver current to the component, and a connection from the component to a GND pin will close the circuit, effectively forcing the current to flow through the component. As you will learn later in this tutorial, all components have a clearly marked pin for input current typically called VCC, and one for connection to ground called GND.

The pins labeled with a D<n> are data pins. These are the pins that are used for communication between the microcontroller and other connected devices. This is going to be covered in later chapters of this tutorial.

The board has two small built-in LEDs (Light Emitting Diodes, or just tiny light bulbs if you prefer) that you can see in yellow in the diagram. The top LED is connected to the pin labeled D4 or GPIO2, while the bottom LED is connected to D0, which is also GPIO16. By the end of this chapter you will know how to turn the LEDs on and off with Python code!

Out of the remaining pins, the A0 is interesting to mention because it is the only "analog" pin in the board. All the other pins can have a binary value, either 0 or 1, but the analog pin can have a range of values, determined by the amount of electrical current passing through the pin. Note that this tutorial does not make use of the analog pin.

Another interesting pin is RST, which can be used to send a reset signal to the board. In future chapters you are going to learn how to use this pin to "wake" the microcontroller when it is in a deep sleep state.

The pin labeled Vin can be used to provide power to the board, as an alternative to the much more convenient micro-USB port. All the examples you are going to build in this tutorial are powered through the micro-USB port, so you are not going to use this pin.

The remaining pins are in most boards reserved for the microcontroller's own use, so there isn't a lot of interesting features to mention there. If you are curious about any use you can give them in your particular board, you should consult the documentation that is specific to it, as these pins are not always wired in the same way.

In the bottom center you have the micro-USB input connector. In this tutorial you will always power the board through this connector. If you connect the other end of the cable to your computer, then the board is going to appear as a serial device in your computer. This serial connection is going to be used to upload code into the microcontroller.

To complete this overview, note that the board has two small buttons labeled RST and FLASH, on the sides of the micro-USB port. The FLASH button prepares the board to be flashed with a new firmware, but this button is rarely needed, as current versions of the board can be put in flash mode through the serial connection. The RST button is, as I'm sure you can guess, a plain and simple reset button, and pressing it restarts the board.