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
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blinding and lots of glare to oncoming traffic
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with a good projector in it.
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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