When developing a project there will come a time that you will need to communicate with the outside world. Serial communication is often the simplest way to get data in and out but when it comes to wireless, Bluetooth is quickly becoming the way to go. Bluetooth modules are getting quite inexpensive and every smart phone has it built in these days. Techbitar has written a great Instructable that goes through the steps to use his Android app that allows you to connect it to your Arduino project via Bluetooth.
Read more about the Bidirectional Bluetooth-based Android Controller for Arduino called Andruino
“ANDRUINO is a simple tool to help you control your Arduino (or clone) from your Android phone. It’s both an Android app and an Arduino program. Andruino has a simple Android user interface to 1) control Arduino’s digital and PWM pins 2) send text commands to Arduino 3) and receive data from Arduino over Bluetooth serial using the ever popular and really cheap HC-05 Bluetooth over serial module or its siblings.”