Their main difference is that they are on different sockets. The i7 9xx are on 1366 and the i7 8xx are on the 1156 socket. You motherboard will have to have a matching socket.
1366 is intel's enthusiasts socket for motherboards and will allow tritple channel memory, 16x/16x crossfire/sli, and better raid configs. Along with upgrade path to the intel extreme 6 core monster. You'll pay a premium for 1366 motherboards however.
1156 is the mainstream/budget socket/platform. These allow you to use i3/i5 as well. They however limit you with a max of 8x/8x sli/crossfire and dual channel memory. These motherboards tend to be less expensive.
Some of the differences between the CPUs themselves are the turbo/power usage.
What are the main differences between the Intel Core i7-930 Bloomfield 2.8ghz and the Intel Core I7-860 Lynnfield?
Getting ready to build a rig and was all set to get the 930, and just found out about a similarly priced 860.
The main difference is that they work on two different sockets. The 930 is on the LGA 1366 and the 860 is on the LGA 1156. The 930 also has a little higher tdp. The 1366 socket can run 2 pci slots at x16 where the 1156 can only do 2 at x8. In game this has minimum effect. The 1366 also supports triple channel memory. The 1156 is technically dead because intel doesn’t plan on building anymore chips on this socket. For the most part there is very little difference that you will notice when you use them.