First off ridge vent does not classify as a gable vent or roof vent. The small slot cut into the top ridge of a roof is all that vents from this product. Ridge vents can be used in addition to gable vents to add ventilation but should never be used as the only exhaust.
A gable vent or wall vent is always placed in the gable of the home no matter the shape of the vent. Wall vents usually have a boxed base for easier install. Sitting a vent into a gable wall with a 2 x 4 sitting flat across the vent opening will allow the vent to rest and remove the weight of trying to hold the gable vent while trying to screw it into place. Also, using screws gives the option of leveling easier by tacking one corner and leveling from there. Screws can be loosened and tightened as well as removed and moved.
Whereas roof vents have an angled base to sit straight on the roof pitch. This angled base prevents the front of the bottom louver from having a gap between the shingles and the base slat. Placing these on some roof flashing to cover the top shingle run will also prevent water blowback in heavy storms. This line does not need sealed if the flashing runs a good distance under the roof vent. Sealing this line can cause water leakage into the attic by blocking drainage or creating a funnel by caulk drying and cracking.
Another roof vent type is a pancake vent and a whirley bird roof vent. These are very limited on the air that can be exhausted. Most even state having 19 square inches of ventillation right on the box. These too should only be used when trying to add ventillation in areas that the ventillation may be blocked from using a gable vent. Attics with interior covered walls and chopped up roofs with many gables or hip roofs may only have this option for some attic areas.
Attic vents and Loft vents only refers to the inside of the attic being vented via an exhaust vent. Whereas roof and gable wall refers to the exterior of the home, attic and loft is just the interior home space being vented. If the vent was labeled as exhaust, it just expells hot air. Intake vents allow cool air to enter a space. Cornice, soffit and overhang vents are usually intake vents. Crawlspace vents are again refering the crawlspace space being vented. These are usually skirting the home's underpinning or block foundation of a conventional floor.