Sometimes having to unlock your iPhone to access your Messages app can be a pain, so Apple has added a quick reply feature as part of its "interactive notifications" feature in iOS 8. This allows users to quickly reply to a message from the Lock Screen without having to unlock their iPhone. Here's how it works.
When you receive a notification on your Lock Screen swipe left on the message to reveal the blue Reply button. Tap the button then type your message in message box and hit Send. Your response will then be sent without having to unlock your phone.
To disable this feature navigate to Settings > Notifications > Messages and scroll down to Message Options. Tap the Show Previews slider so it appears White.
You can also pull down on a notification to reply to a message from anywhere on your phone. This will also open a message box that you can use to quickly respond to a text message without opening your Messages app.