1366 (X58) is a 3 chip solution and 1156 is a two chip solution.
1156 (P55) moves the x16 PCIe bus onto the CPU chip package, but the downside is there is only x16 PCIe lanes available.or two x8 PCIe lanes. The good point is latency is lower and costs are kept down.
The other chip on 1156 (P55) is called the PCH (Platform Controller Hub), this handles SATA2, HD sound, USB2, Firewire, Lan, and all other peripherals. Interconnection between the PCH and CPU is handled by a 2GB/s DMI.
1366 (X58) also supports tripple channel memory, while 1156 (P55) supports dual channel memory
There is much more to it than this, but that is the basics.
Maybe these diagrams help.