it's the type of LCD used.
HPA is a passive technology, and there is ghosting, that means if you move something on the screen, it blurs which is bad. TFT is active, which means less ghosting.
HPA is also somewhat darker than TFT. overall TFT is better for your eyes. um, HPA is cheaper than TFT, so you see passive screens on the cheapest of laptops.
since your eyes are important, look for active screens no matter what. that's what i do. if it's not active, it's not on my list. though if it's cheap enough, i'd go for it... ;-)