What is the difference between an amateur and a professional camcorder?
I'd regard this as off topic as it concerns video cameras not stills, but anyway a lot of professionals use the Red series of cameras:
http://www.red.com/
The most popular being the Red One:

(source: redoneutah.com)
Benefits over consumer level video cameras nicely mirrors the difference between DSLRs and Compacts, namely:

The ability to shoot video in raw, for example using the proprietary RecCode format. This makes contrast adjustments, post production and grading easier.

A larger sensor, the Red One has a similar sized sensor to a DSLR (same size as super 35 film) this allows better low light performance and shallower depth of field.

The ability to have interchangable lenses.

Higher resolution pictures (again to refer to the Red One it will output at 4k (4,096 x 2,304 pixels, which is at least four times full HD)).

Better build quality. All professional tools should be built to be packed up and shipped round the world and work first time on location.


There are other advantages specific to professional videography cameras, namely:

Higher framerates for [real] slow motion 24fps -&gt; 1000fps (some consumer cameras do this, but not very well)

Wider range of inputs for mics etc.

Wider range of accessories, HD out for live monitoring etc.

Record to multiple data banks, HDDs, flash memory etc.


Another difference is that professionals will more often than not hire equipment as the cameras cost more than even the most expensive 35mm DSLRs, and they are typically only shooting for part of the year in intensive bursts. This also enables the use of specialist cameras (such as the Phantom 1000fps camera, which can only be rented, not bought).
It's worth pointing out that DSLRs which shoot video offer many of the advantages posted above, but they lack the modularity and flexibility of output that you get with a professional digital cine camera. They are also limited in terms of shooting time.