iMessage is Apple's built-in instant messaging (IM) service. It lives inside the Messages app on iPhone and iPad. On the iPhone it works alongside — and often replaces — carrier SMS and MMS. On the iPad provides similar functionality, allowing you to send text, picture, video, voice memo, location, contact, and other types of messages to anyone else using an iPhone, iPad, or Mac. Since there's no charge for iMessage beyond data (cellular or Wi-Fi), you can send unlimited iMessages pretty much for free. Take that, carriers!
iMessage in iOS 8
Messages in iOS 8 greatly increases the functionality of both iMessage and SMS/MMS. New touch-and-hold controls have been added allowing for quick and easy video and voice messages to be sent. Messages can be heard by simply raising the iPhone to your ear. Sharing recent photos and videos is also easier than ever, with big thumbnails and one tap sending.
Group messaging has also been vastly improved. Ongoing conversations can be given a name. You can go on Do Not Disturb on a per-group basis, and you can leave them any time you like.
Location sharing, for an hour, for the rest of the day, or in perpetuity, can be done directly inside of Messages. There's also a new Details view that shows you not only the location of everyone sharing it, but all the photos and videos that have been sent as part of the conversation, all together, all in one place.