As far as I'm aware
"U" Stands for Ultra-book (slim, lightweight)
"N" Stands for Non-glossy (matte, non-reflective screen)
"G" Stands for Gaming
I don't know what the letter K actually stands for, but it seems to cover just about everything else. The rest of their model numbering system refers to other variables, like screen size, processor, hard drive etc, but they're not so easy to de-cypher.