iMessage is Apple's proprietary service that allows you to send and receive free, SMS- and MMS-like messages on your iPhone, iPad, iPod touch, or Mac from any other iMessage user. Not only can you send regular text, photo, and video messages with iMessage, but contacts, voice memos, and locations as well. iMessage works on any device running iOS 5 or higher, and any Mac running OS X 10.8 Mountain Lion or higher. Whether you need help setting up or using any or all of the features, you've come to the right place!
Note: Updated for iOS 7 and OS X Mavericks 10.9.2. We'll be updating this guide regularly as Apple introduces more iMessage features, so make sure you bookmark this page and check back frequently!