A 'chopper' is a motorbike that has been stripped of all not absolutely necessary parts (plastic chassis, aerodynamic parts and fancy metalwork for example).
The name came into being after the Second World War, when the returning GI's bought up war surplus motorcycles, and literally chopped off the bits they did not want. It is also the name of a late Seventies bicycle, with the 'chopper' handlebars.

Many bike companies like Honda or Kawasaki now call their recent chopper models 'custom' or 'individual classic'. A 'cruiser' is a bigger, havier variant of chopper designed for long journeys and resembles a modern touring bike with the looks of a classic chopper.

It is not build to drive fast but to cruise at about 80 Kph down the country road.
And it is build to be noisy, because driving loud is fun according to chopper riders. The best known chopper is the classic Harley Davidson Low Rider. This model virtually started the chopper cult in the Fifties. Many 'hardcore' bikies build their own choppers by stripping touring bikes and modifing them (especially the muffler system to achieve a better sound).

A chopper is your choice of bike, if you want to take it easy and have fun on the road. Riding chopper is a great way to relieve stress and tension especially if you ride together with a few mates.
Hence the biker saying 'Life to ride, ride to life... Free spirit'.

The chopper that is currently one of the nicest looking ones, is the 'Black Widow' from Honda Motorcycles. The previous model 'Shadow' had a nice optic, too, but the 'Black Widow' looks much bigger and cooler. The 'Black Widow' is totally black with a lot of chrome. That is, all the engine parts, cylinders, exhaust pipes, mufflers, handlebars and wheels are chrome. It looks real good, but is a lot of work, if you have to wash it all, especially after you had a ride in the rain. The 'Black Widow' has spoke wheels as has any other 'real' chopper and that is also a pain if you have to wash it, but otherwise it looks real groovy. It has a big wide tank with instruments in the Harley style (only a speedo on the tanktop, no rev-metre or fuel gauge) and a real nice short, backswept handlebar. All in all it looks like a real classic chopper.
Now, many people will say that it is not, because it comes from mass production but I don't mind that. I also don't mind that it is no Harley. I love Harleys, they look good, sound good, drive wonderfully, but you always have to maintain them. If you are an experienced motorbike freak or automechanic, that's no problem, but it is very hard for the beginner. That's were Hondas or Kawas come in: They are extremely reliable, you don't have to fasten the screws all the time, and these days they also look good!
But of course, with choppers as with any other things on this planet, beauty lies in the eyes of the beholder. I generally like all choppers, and I think it is the only motorbike to ride. It gives you an unique feeling of freedom and peace on the road.