What is the difference between cruiser and chopper bike?

Harley Davidson a a pure Cruiser motorcycle.

Royal Enfield Thunderbird Twinspark is the only cruiser bike model from Royal Enfield bikes and other models are commuter bikes.

A chopper is a category of motorcycle which are specially custom built according to personal preferences from an original motorcycle design or built from scratch to attract peoples.
The chopper bikes have extended forks which are lower to the ground, wider tyres and are more artistically designed to attract crowds than normal bikes.
The worlds best Chopper bike designers are Orange County Choppers.

Cruiser bikes are mainly designed for longer rides and have more extra comfort in riding and handling, have very powerful engines and have solid muscular looks and are heavy.
The best examples of cruiser bike models are Harley Davidson, Triumph Rocket III, Victory etc.