What is the actually difference between the Intel I5-6600 and the I5-6699k?

[JStringfellow]

What is the actually difference between the Intel I5-6600 and the I5-6600k?
I am asking out of curiosity because I ordered the 6600 by accident thinking I was getting the 6600k. I am considering sending it back, but if the performance isn't too drastically different then I won't worry about it. I plan on gaming with it.

 

[littleleo]

The i5-6600K runs at 3.5GHz and is unlocked for overclocking, and doesn't come with CPU fan. The i5-6600 runs at 3.3GHz is locked and may come with CPU Fan. The I5-6699k doesn't exist.

 

[Moldoror]

I5-6600k - Unlock CPU. You can OC easy
I5-6600 - Lock CPU, Hard to OC.

Thats all xD