GDPR vs Privacy Shield
The General Data Protection regulation is a set of laws due to be enacted in the EU in 2018.
Privacy Shield is an agreement between the EU and US allowing for the transfer of personal data from the EU to US.
The GDPR has specific requirements regarding the transfer of data out of the EU. One of these requirements is that the transfer must only happen to countries deemed as having adequate data protection laws.
In general the EU does not list the US as one of the countries that meets this requirement.
Privacy Shield is designed to create an program whereby participating companies are deemed as having adequate protection, and therefore facilitate the transfer of information.
In short, Privacy Shield allows US companies, or EU companies working with US companies, to meet this requirement of the GDPR.