Hi -
The "K" is Intel's designation for an "unlocked" cpu. This means that Intel has enabled access to all of the levers, switches, dials, and buttons that allow you to have control over the CPU innards for overclocking.
If you plan to overclock, then you want the K version. If you think you might want to overclock, maybe, at some point in the future, then I'd still get the K version so that you have that option.
If you positively, absolutely, will never ever overclock, no sir, no how, no way, then go with the non-K.
Yep, you'll also need a Z87 chipset mobo, like was mentioned in another response.
Hope this helps.
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