Basically the same as a seebeck generator; but it can be used in places where current seebeck generators would not do well.
I wish they would phrase these things as "turns heat differences into electricity;" because every time they say "waste heat," people get excited about harvesting heat from their CPU and having their computer power itself.
Well in theory a proportion of the heat generated by a hot running CPU or GPU (or anything else that operates at a temperature significantly higher than the local ambient temperature) could be converted to useful energy, and you could feed the generated power back into the unit to reduce what it needs to sink from other sources.
Though of course you would have to work out if the energy savings are enough to outweigh the extra energy used in manufacturing such a device (remembering to account for the fact that the extra complexity of each unit may reduce the effective yield of the manufacturing process). It might cost far more energy to make than it'll ever save in its active life. Thermodynamics can be a bugger like that...
I'm going to play a bit of "devil's advocate" here. :-) The general problem of place-shifting and time-shifting energy (wind power / solar power) is a very real issue and you could make tons of money solving it.
I wish they would phrase these things as "turns heat differences into electricity;" because every time they say "waste heat," people get excited about harvesting heat from their CPU and having their computer power itself.