hid = those really bright blueish lights that newer cars have.
retrofit = putting Projectors from higher end cars into YOUR headlights to make it better no blinding, and no glare.
hid in your stock headlights looks like this
blinding and lots of glare to oncoming traffic
with a good projector in it.
with halogen projectos (notice the the ghosting above the cut-off)
doing a retrofit will set you back a bit 600-1000 depending what projectors you want, and other stuff
go to hidplanet.com to learn more