K = Kelvin. This is the temperature colour of the light that the bulbs emit.
So different temperatures produce different colours. A typical OEM colour is 4300K. The newer ones are closer to 5000K. 4300K is slightly yellow whereas 5000K has a tint of blue that gives it a distinct white look. However this actually produces less useable light, particularly in poor conditions. So manufacturers have had to bump the power up to compensate.
The thing to note with aftermarket HID kits is that the colour temperature is not accurate. Because a lot of people think that the higher the colour temperature, the more light they will get, manufacturers have increased the temperature rating for better sales. So you could buy a 6000K bulb but it is actually in fact only 4300K or 5000K.