Saltwater jigging lures or metal jigs are made for vertical fishing in deeper part of the sea; where other technics are less productive. Usually the bigger fishes stay near the bottom during daytime, only way to catch them, it's presenting a lure directly to them. Pelagic fishes don't like light, prefer deeper area, with less light & heat, bottom fishing or jigging is the most productive technique for these large fishes.
Jigs are usually made with metal, weight between 100 to 300 grams. Mainly, there's two types of JIG on the market, "Center weight" or "tail weight". On "center weight", the weight is balance around the center of the jig, on the "tail weight" they are placed towards the tail. Center weight jig is mainly used for midwater fishes (30-70m), slow descent with sideways movements. Really deadly on Cudas, kingfish, sailfish.
The "Tail weight" jig is balanced with a weight near the tail. This jig is designed to reach bottom quickly and lift with a less action. These jigs are used for deeper fishes. They are more streamlined, thinner, provide less resistance in the current and require less effort to manipulation.
Long or shorter jigs? your choice will depend on current's strength and the depth of your fishing area. Heavy lure will drop faster and will stay more vertical, for against, it will be harder to handle and require more effort. If it is too light, it will take more time to reach bottom and will have a open angle (current), but it will be less physical. The jig length will also affects the descent of the lure, for against, a big lure takes bigger fish. If you fish from 90 to 130 meters deep with a moderate current, a 150 to 230 grams jigs will be perfect.
Brand: River2Sea, Braid, HqJig, Shimano
