These two processors are codenamed Devil's Canyon and they differ in several ways from the regular Core processors. These chips are made from high-performing dies binned out from the foundry, they are placed on extra-durable packages with contact points designed to handle higher voltages and they also feature a superior thermal interface material between the die and the integrated heatspreader (IHS).
The Core i7-4790K will be Intel's first processor to have a stock clockspeed of 4.0GHz and a Turbo Boost of 4.4GHz. The Core i5-4670K on the other hand has a stock clockspeed of 3.50GHz and a 3.90GHz Turbo. Both chips have a 88W TDP, 4W higher than the non-K chips.
Both processors are expected to be introduced at Computex 2014 with availability slated for around mid-June. Devil's Canyon may be quite an exciting chip for overclockers, if the rumors are right they may be capable of running at 5GHz on air cooling.