Hi,
Best thing is you do a search here for the word 'carnet', but here is a quick description.
The carnet is a customs-document for your bike. It guaranties the country you enter that you will pay the import-duties if you do not lave in time.
For that you need to deposit a sum of money equal to the highest import-duty for the countries listed on your carnet. For example: India and Egypt require 300% of the value of the bike.
You need a carnet for most of Africa and most of Asia. Australia is easier with a carnet. You do NOT need a carnet for south, central and north America and Europe.
You get a carnet at the AA of your country or the country that the bike is registered at.
The cost of the carnet depends very much on the sort of guaranty. If it is purely a deposit the carnet is cheaper but the deposit is higher (up to 10,000 U$)
If it is a insurance-combination, the carnet is more expensive, but the deposit is a lot lower or sometimes there is no deposit at all.
As for Andy's wikipedia info, The carnet was created to stop customs and police to do shabby business and to make things a lot more easy for travelers.
Do you honestly expect that if you pay a customs-official in lets say Zambia a deposit for the import-duties, you will get that money back if you leave the country? or ever? Even for a 'honest country' like Australia it is a hell of a job to get a import-warranty refunded. Very often this process costs more then the deposit itself.
As for "problems" with the carnet, as long as you can prove you still have the bike with you (at the end), not one country can run of with your money.
For more info, just search the HUBB. There are a zillion posts about it.
Maarten