One of the useful smaller features added in iOS 8 is the ability to quickly read and reply to your Messages directly from the new message notification that comes up.
When a new message comes in while you’re using your iPhone, the notification banner that appears can actually be “pulled down” to read the full message and quickly reply to it; simply tap on the banner and swipe downward to “pull” the banner down and the rest of the message text will appear (for longer messages), along with a reply field. Even if you don’t want to reply right away, this method has the advantage of allowing you to read the full text of longer messages. Be aware, however, that if you have Read Receipts enabled, doing this will mark the message as read, sending a “Read” notification back to the sender, even if you don’t open the Messages app.
This also works from the lock screen and Notification Center. Swipe right-to-left on the notification and a “Reply” button should appear that you can tap to bring up the reply view right away without the need to unlock your iPhone or go into the Messages app.
Note that if you have disabled Message previews, you won’t be able to reply from the lock screen, as this would reveal the content of the message. However, you can still use this quick reply feature from the Notification Center (once your iPhone is unlocked) or from the notification banner if a message comes in while you’re using your iPhone. Although the notification banner will still only say “iMessage,” pulling down on it will reveal the text of the message, saving you a trip into the Messages app if you’re curious to know what the sender has to say without having to interrupt what you’re doing.