Sandy Bridge (2011) 15" Macbook Pro:
1440x900 glossy display by default, optional 1680x1050 glossy or antiglare display.
Sandy Bridge quad core processors and AMD graphics
Thunderbolt port, USB 2, Ethernet, Firewire 800, DVD burner.
Ivy Bridge (2012) 15" non-retina Macbook Pro:
Same as above except for Ivy Bridge quad core CPU, Nvidia Kepler graphics (GT 650M), and USB 3.0.
Ivy Bridge (2012) 15" Retina Macbook Pro:
2880x1800 'retina' glossy IPS display.
Ivy Bridge quad core CPU, Nvidia GT650M graphics, solid state flash storage by default, two Thunderbolt ports, USB 3.0, HDMI port.
Can drive 3 external displays in addition to the built in display (the others above can drive 2 external displays, but only by using Apple's Thunderbolt display which supports daisy chaining)
Option to change desktop scaling to provide more or less screen real estate while using high quality scaling. The highest option gives you the equivalent screen 'real estate' as the old 17" MBP.
Thinner than the non-retina Macbook Pros, and about a pound lighter.
Ethernet and Firewire require adapters. No built in DVD drive. RAM cannot be upgraded after purchase.