Flatbed trailers are the industry standard for open tops. Standard flatbeds are versatile enough to carry practically any kind of load, even extremely tall loads (like the oil equipment used in Alaska) that doesn't have a chance of making it under a bridge. Flatbeds require straps to secure any kind of freight, and may also require a tarp depending on the type of freight and local jurisdiction.
Drop decks (a.k.a. "low-boy" trailers) are just like flatbeds, but have a dropped-down section in the middle. Drop deck trailers not only make loading and unloading easier (especially when side-loaded), but they may be the only option for delivering certain types of very tall cargo. Construction equipment is often delivered on drop decks, since loading such equipment on a standard deck would put it well over the 13-foot, 6-inch maximum allowable on interstates.
Logging trailers are essentially oversized "stake-side" flatbeds. Logging trucks utilize several heavy duty steel uprights on either side to help keep logs in place while shipping. These steel uprights must be incredibly strong to resist pushing outward under load. Some logging trailers even have their own lifting claw (crane) to lift the logs and load them onto the trailer. Because the crane adds so much weight to the trailer, such all-in-one units are generally only used to transport materials around the logging site or on short hops on overweight-permitted roads.
Dump trailers are used in place of standard dump trucks to transport larger loads of material. Dump trailers have all the same hydraulic mechanisms as standard dumps. Because standard dump trucks can legally carry well over 40,000 pounds of cargo, trailer dumps are more often utilized where the freight is large in volume but fairly light (like garbage or hay).
Bulk trailers are similar to dump trailers in that they consist of a large bucket loaded from the top, but don't have the rear swing-door or hydraulic mechanism that define dumps. Instead, bulk trailers are pointed at the bottom center (earning them the epithet "pregnant guppy"), and use a retractable slider-door to unload their contents. To unload a bulk, the driver parks over a receiver pit, opens the slider door and waits for gravity to drain the trailer. Bulk trailers can be divided into two, three or four smaller containers to transport different kinds of bulk in one load. Bulk trailers are often used to transport grain stocks (like corn and wheat) and construction materials like sand and gravel.