This concerns the recently-released Intel Haswell CPUs. These are marketed as "4th-generation" Core processors and typically have 4xxx part numbers.

K-series CPUs are unlocked so they can be seriously overclocked. But beware of features present on the standard chips, that are disabled on the K-series:

"Paying the extra for a K-series product also means giving up support for one of Haswell's key features, the TSX extensions that enable transactional memory. Intel has stripped out the VT-d device virtualization and vPro management features in the K series, as well."

These features are useful if not critical if you're running a virtual machine manager (VMM) and virtual machine(s) on your system. Examples might be VMWare, OracleVM VirtualBox, Hyper-V, Win7 XP Mode etc. You might be better off staying with the standard chips if that's important to you.

http://techreport.com/news/24950/intel-removes-modest-free-overclocking-from-standard-haswell-cpus