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.
The "non-K" versions do have a few items that appear to be more interesting, particularly in a corporate environment running virtual machines. It looks like the assumption is that home users who want to over-clock are probably not running multiple VMs on their hardware.
But coming back to the point of the question, ItssSteve, what are you trying to accomplish? Or, what prompted your question in the first place? We can have a great technical discussion on "-K", but I want to be sure that we're actually leaving you with information you can use to solve your problem.
No. The "S" is a lower power version, but generally speaking "non-k" version have additional security features that prevents "information snooping" in critical areas of the cache.
Seems weird that they would leave those features out of the more expensive version...
Businesses are more inclined not to overclock and those features that I described more useful to them. Individual people (especially gamers) would tend to overclock, but those security features are no use to them because people do not have their own IT department to connect to when having technical issues and they do not deal with highly sensitive real time data stored only in the cache like financial institutions and highly classified information like federal security agencies.