Libraries are pieces of code that commonly add functionality for specific pieces of hardware (can be software functions). Instead of putting all that code into your sketch, you can include and call a library that has the functions you need, passing it parameters for your specific implementation. This keeps your sketch uncluttered, and can help you use hardware that you may not understand. The Arduino IDE comes with lots of libraries built in, but it is common to have 3rd party libraries included with various pieces of hardware. In the past, you would manually unzip these libraries and copy them to the appropriate folders (and you still can) but recent versions of the IDE have a import library function that accomplishes this task for you. Read more at http://arduino.cc/en/Guide/Libraries
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