The Mac Pro "Twelve Core" 2.66 (Mid-2012/Westmere) features dual 2.66 GHz Six Core 32-nm Xeon X5650 (Westmere) processors with a dedicated 256k of L2 cache for each core and 12MB of "fully shared" L3 cache per processor. It also can be equipped with other dual Six Core processors, but these are documented as distinct models for convenience.
By default, this model is configured with 12GB of 1333 MHz DDR3 ECC SDRAM, a 1TB (7200 RPM, 32 MB cache) 3Gb/s Serial ATA hard drive, an 18X dual-layer "SuperDrive" and an ATI Radeon HD 5770 graphics card with 1GB of GDDR5 memory.
Expansion includes two external 5.25" "optical" bays (one available by default), four internal 3.5" "cable-free, direct attach" hard drive bays (three available by default), and four PCIe 2.0 slots (one free PCIe 2.0 x16 slot and two free PCIe 2.0 x4 slots with the default single graphics card installed).
Port selection includes five USB 2.0 ports, four Firewire "800" ports, dual Gigabit Ethernet, two Mini DisplayPorts and a dual-link DVI port, and more. AirPort Extreme (802.11a/b/g/n) and Bluetooth 2.1+EDR comes standard.
Please note that the "Mid-2012" Mac Pro models practically are identical to the "Mid-2010" models before them, but there are some differences in terms of specs and identifiers and EveryMac.com has documented them as distinct models, accordingly.