Core i7 860 or 870

I was looking around for a new processor and I noticed that the core i7 870 is well over 500usd while the 860 is under 300, the 870 is clocked a little higher at stock but why such a huge price difference?? is it more powerful? If you overclock the 860 to 870 clock, will they perform the same? The same is with the 920 and 940 i7 processors.

Also can someone explain to me what TDP means, like when in specifications it says "95w processor" or "130w processor"

Thanks.

Honestly, there's not nearly enough difference between the pairs of processors mentioned to make the higher ones (870/940) worth it.

Keeping it simple, thats the power/heat load that the cpu will place on the system at stock settings.