Ear Defenders are basically just a plug that helps reduce ear damage from sudden loud noises - they are useful on places like rifle ranges etc. Stick your fingers in your ears, it will produce a similar effect - you can still hear things but not so loud (even people talking)
Noise cancelling earphones are different. They produce a noise that actually cancels out the sounds in the environment. Sound waves have peaks and troughs - Ear cancellers produce opposite troughs and peaks and the combination cancels each other out - reducing noise. They are good for situations where you are exposed to noise for sustained periods of time - such as on an airport apron with jet engine noise.