The K on the Intel® Core™ processors is used to show that the processor has an unlocked multiplier. By unlocking the multiplier it allows easy overclocking of the processor. Since overclocking is generally through to be an enthusiast feature we have disabled some features on the Intel Core i5-2500K and Intel Core i7-2600K.
The differences between the Intel Core i5-2500 and the Intel Core i5-2500K.
Intel Core i5-2500
-TXT (Trusted Execution Technology) a business level virus protection feature.
-VT-d (Virtualization with Directed I/O) allows you to assign hardware to VMs
-Intel HD 2000 Graphics
When a single core is active, the chip can turbo up to 3.7GHz. If you want, you can change that turbo state to go as high as 4.1GHz. Overclocking these chips relies entirely on turbo however. In the case above, the fastest your chip will run is 4.1GHz but with only one core active. If you have four cores active the fastest your chip can run is 3.8GHz
the Core i5-2500K and Core i7-2600K are fully unlocked and will let you overclock them as far as the CPU and/or your cooling can sustain.