Those satellites, and the other two that will be launched in January 2012, will be used for the In Orbit Validation - IOV phase of the Galileo Project. "This quartet of satellites, built by a consortium led by EADS Astrium Germany, will form the operational nucleus of the full Galileo satnav constellation. They combine the best atomic clock ever flown for navigation – accurate to one second in three million years – with a powerful transmitter to broadcast precise navigation signals." ESA says.
The launch will use a Soyuz launcher.
For this historic moment, ESA set up a dedicated website with all the information available.
IOV dispenser (photo from ESA website).