There are U, T, and K series processors with the i5 (for instance) range, U's are very low power (mostly laptop), T's a low power (35W typically) with lower clock speeds, no letter is normal power 74W I think, and K is normal but with the ability to overclock. So you normally need no letter or K, unless you are trying to build a very low power rig (if you are constrained to a 100W PSU for instance.)