I will try to answer in detail as possible.
Customs duties apply to all imports under which imports are usually free from duties. Sometimes you may be lucky enough to have small imports slip through without being slapped with a fee, however you certainly can't take this for granted.
If you are importing a small amount, then many couriers will automatically add the duty into the shipping price. However, some shippers such as EMS don't have this practice, making considerably cheaper to ship through them.
If you are importing a large amount (e.g. by the container load), then you will need to employ a Customs Broker and/or Freight Forwarder to take care of your import for you.
Finally (and very generally), most duty rates fall between 2% and 30%. Items like books usually have a very low tariff, while shoes and clothing often attract some of the highest tariffs.
You may want to look into this link to learn more about customs: Link hidden: Login to view
Also here are links that provide you advice on how to deal with Chinese suppliers: Link hidden: Login to view
Hope this helps!