The ESP8266 is a microcontroller with integrated WiFi. The main difference between a microcontroller and something like the CHIP or RasPi is that they are much

The ESP8266 is a microcontroller with integrated WiFi. The main difference between a microcontroller and something like the CHIP or RasPi is that they are much less powerful and are designed to run a single program instead of running a general operating system like CHIP does. They're what you use for single-purpose devices like a weather station or a light switch, while I think of small computers like CHIP as being better suited to serving as the brains for controlling sensor networks and such. "Arduino" is used to refer both to a family of microcontrollers based on an Atmel processor as well as a set of libraries designed to make programming microcontrollers accessible to people without embedded hardware experience. Originally, the ESP8266 was marketed as a simple serial Wifi adapter for the (hardware called) Arduino, but the community quickly realized that they could be programmed on their own and used to replace the Arduino. Subsequently, the Arduino libraries were ported to the ESP8266, making it instantly much more accessible as well as giving it access to a huge range of libraries for interfacing with different hardware and software. ESP8266 refers specifically to the processor, but it is usually sold in an integrated package with onboard memory and antenna etc. There are several of these packages for different uses, called e.g. ESP-01 or ESP-12e. While you can use these modules yourself with a bit of knowhow and some deft soldering, much easier is to buy a prepackaged dev board like the NodeMCU or WeMos D1. These are basically the equivalent to a CHIP (albeit again much fewer IO pins)- they pair an ESP integrated module (usually the ESP-12e) with all the supporting hardware needed to power and program the module. You just plug in a USB cable, load your code on and connect whatever external devices to the GPIO pins. I didn't have any significant electronics knowledge when I dove in head first. Arduino is designed to be accessible and there are plenty of kits with detailed instructions and all the parts you need. The ESP8266 has been wildly popular in the last year or so because it's got Wifi + compatible with Arduino (i.e. easy) + dirt cheap, which was previously unheard of.