iMessages are photos, text messages or videos that you send to iOS devices and Macs over Wi-Fi or Cellular networks. They are encrypted and your message will appear with a blue background.
iOS Key Features
The Notification Center is what you see when you swipe down from the top of your screen. You can see alerts, notifications, reminders, etc. It is all based of what you have turned on under your Notifcation Settings as to what you will see.
Lock screen on your device is referring to the screen you see when your device is locked. It will ask you for your Passcode if you have that enabled.
Emoji refers to the icons that you can use on your phone such as a smiley face, flower, christmas tree, etc. Emoji's are a fun way to show expression without using words.
The Home button on your device is referring to the button that you have on the bottom center of your device that you can push to get taken to your Home screen. It is also used when you power off your device or when you press/hold you can access Siri.
Accessibiity refers to things that are accessible to you on your device that helps make things easier.
The Control Center is found by swiping up from the bottom of your screen on your device and it has all of your most used features located on it such as Wi-Fi, Bluetooth, Display Brighness, Volume, etc. There are more option such as music that you can access by swiping to the left on the Control Center. It is like home base for your device.
3D Touch is available on newer iPhone's and is a capability that you can use where you press down a slight bit longer than normal while pressing on the screen of your device and you will get immediate options. For example, if you hold down on the phone icon you can get 3 options of people to call that you have spoken to last. 3D Touch is best described, as a timesaver.