I noticed that the Nikon D4 uses XQD memory cards along side Compact Flash memory cards. What are the differences or advantages of XQD?
XQD is an open format created and licensed by the CompactFlash Association. The interface is based on the PCI Express or PCIe specification, a standard for serial interfaces. The specification was announced on Dec 7th, 2011 in this press release.
The first camera to be announced that uses the format is the Nikon D4. Nikon claims a write speed of up to 125MB/s or 1Gbps in the D4, although this has not been tested independently as of yet. They also claim up to 125MB/s or 1Gbps read speed.
Currently, only 16GB and 32GB capacities are available, through the Sony H Series of XQD cards.
Other specifications noted by the CompactFlash Association:
As a comparison to Compact Flash, the XQD format isn't necessarily faster, but the interface has the potential to be much faster. For example the Lexar 1000X CompactFlash card claims a read speed of 150MB/s and write at 145MB/s, but current cameras can't keep up with this speed as most do not support UDMA7. The Nikon D4 according to the Sony specs released, can keep up at 100 continuous RAW shots.