Reflectors are used to direct the HID bulbs light downward toward the garden. Some reflectors are equiped with vent flanges on each end to be used to remove heat caused by the bulb, a very popular item for those who garden during the summer and need to reduce daytime temps.
Horizontal Reflector are used when you want to have the bulb burn horizontally, in some cases these hoods are lower rofile saving you valuable room.
Vertical Reflectors are used to burn the bulb vertically, these hoods can not be vented with a fan like some of the horizontal reflectors. Although it can be argued that the parabolic hoods provide a larger and more evenly distributed light, which can turn into larger yields.

Use parabolic hoods when the added heat will not matter (ie. extremely well vented or large situations) or during winter when the heat can be used to warm the grow area. Use horizontal vented hoods with an attached exaust fan in the summer to keep the grow area at optimal tempuratures. If you utilize AC units during summer weather you can save a lot of electricity by using vented hoods during the hot weather, removing the excess heat with a exhaust fan instead of an expensive air conditioner could save you hundreds.

COOL SUN REFLECTOR