Trolling is generally used for big game fishing. It consists of dragging a lure behind the boat in order to get the fish to eat.
The lure can be of a big size like for blue marlin or giant bluefin tuna. It can be of various colors, sometime more to atract the fishermen rather than the fish. Lures are usually artifial, but they can be also a live bait or dead bait.
You may do the trolling at various speeds, fast trolling can be done for wahoo and yellowfin while other species require a slower speed such as seven knots. You also can do slow trolling for other fish when in the area specially when you use live bait in order not to kill the baitfish.
There can also be a mix of a dead baitfish and skirt. These are called skirted lures. Lures can also be trolled deeper under water in order to get closer to the fish that are on the bottom. You can troll over wrecks or underwater reefs which is where the fish tend to be. For this tipe of trolling you need a good fishfinder in order not to get your lure accidentally caught on the bottom and a downrigger to get the lure exactly as deep as you like. This bottom trolling is usually done very slow.
The speed of any type of trolling also has a lot to do with weather conditions. The idea of surface trolling is to make your lure seem like a real fish to the predator, it should go in and out of the water and create a smoke trail to make it seem like its getting away. For me I tend to use darker lures on cloudy days and more colorful lures on sunny days.