iOS is the operating system that runs on Apple devices such as iPhone and iPad. The user interface of iOS is based on the concept of direct manipulation, using multi-touch gestures. Interface control elements consist of sliders, switches, and buttons. The response to user input is immediate and provides a fluid interface. Interaction with the OS includes gestures such as swipe, tap, pinch, and reverse pinch, all of which have specific definitions within the context of the iOS operating system and its multitouch interface. Internal accelerometers are used by some applications to respond to shaking the device (one common result is the undo command) or rotating it in three dimensions (one common result is switching from portrait to landscape mode).
iPhone is a smartphone that can function to text messaging, third-party launch applications (a.k.a Apps), play music and video, take a pic as a camera phone, etc.
iPad is a line of tablet computers, which unlike them, it can do many things like reading books and web browsing using just a touchscreen.
iPod is similar with iPhone, but it is much thinner and can't make any phone call or other non-internet communication