Lowboys are called that because the bed is of a 'drop center' design. This means that the bed is lower in the front of the rear wheels and then rises sharply at the front to reach up over the rear of the truck. This front section which bends up over the rear of the truck is called the "goose neck'. This type of trailer is used commonly for heavy hauling because you can get a larger load closer to the ground than would be possible on a 'flat bed' trailer so called because the bed is 'flat' and extends from a point over the rear tires forward over the fifth wheel.
The challenge of a lowboy, of course is getting it loaded or unloaded. There are more or less 3 ways to load/unload a lowboys- over the back, over the sides or over the front. Shown below is a gooseneck lowboy. the equipment to be carried drives up on to the deck after the trailer is lowered to the ground. After loading the gooseneck allows the tractor to back under it and lift the load onto the fifth wheel.
Another example of a gooseneck lowboy, on the ground. Note this one has 3 axles vs. two axles at the rear on the one above. When really big loads are carried, extra jeeps, boosters, or trucks can be added to increase the number of axles and tires and distribute the load over a greater area.
A lowboy carrying a skip loader.
For additional discussion about lowboys: