Gold filled jewelry is made by fusing a layer of gold and base metal. Because of the more intensive bonding process than gold plated jewelry, gold filled pieces have up to 100x more gold alloy than gold-plated pieces.
For this reason, gold filled jewelry is more expensive than gold plated, and it is safer for people with skin allergies. Because of regulations, the US has set for labelling pieces as gold-filled. You will always be notified if your work has been labelled as such when shopping for jewelry.