While vanilla Arduino can be very useful and an awful lot of fun to learn, I knew from the beginning that I wanted to connect my Arduino (and other devices) to the Internet so they could be queried remotely and log their data no matter where they might be located physically. Once you get beyond the basic included Wifi or Bluetooth included in some boards, like my Arduino Yun or the Arduino 101, things can get complicated pretty quickly. This interesting article from HackADay lays out the current state wireless tech you can use with your projects including WiFi, Cellular data, RF radios, mesh networks, products, devices and more. It provides a great underpinning to build on in your own projects.
It seems these days all the electronics projects are wireless in some form. Whether you choose WiFi, Bluetooth Classic, Bluetooth Low Energy, ZigBee, Z-Wave, Thread, NFC, RFID, Cell, IR, or even semaphore or carrier pigeon depends a lot on the constraints of your project. There are a lot of variables to consider, so here is a guide to help you navigate the choices and come to a conclusion about which to use in your project. We can really quickly reduce options down to the appropriate tech with just a few questions.