Before today, Apple's high-density Retina Display only existed on its iPad and iPhone products. With the introduction of the 2012 MacBook Pro with Retina Display, that feature is now available on the laptop as well.
In addition to a much denser display, the Retina Display MacBook also boasts a discrete NVIDIA GeForce GT 650M graphics card with 1GB of GDDR5 memory and automatic graphics switching. That's two reasons it needs a beefy 85-watt power supply (below), not the 45-watt model found in MacBook Airs.
Another comparison: the Retina Display MacBook (top) compared with an earlier MacBook Pro (bottom). There are no dedicated FireWire or Ethernet ports on the Retina model, but Apple is offering Thunderbolt-to-Ethernet and Thunderbolt-to-FireWire adapters, sold separately.