This was posted on another site by a friend of mine, and with his kind permission, will be posted here for you education to continue.
A lumber tarp, in my experience, is 25' long, by 24' wide. It can also be referred to a tarp with "8' drops". The 8' drops comes from the fact that your trailer is 8' across, so if your tarp is centered on your load, you will have 8' of material hanging down on each side. These kinds of tarps are good for lumber, wall board, trim molding, and other "tall" loads that you will have on your trailer. That way, when you go to secure the bottom edge of your tarp, you dont have to roll up 4' of tarp along the bottom. Think of making a twin size bed with a king size sheet. There is alot to tuck under.
Now, some lumber tarps have either a flap at the front end, or a "boxed end". If it has a flap, the flap is generally 12' wide, and 8' tall, sewn across the front middle of the tarp, so you can fold it down over the front of your load. Its 12' wide so you can lay it don across the front of your load (lets say lumber for example) and wrap the corner edges around the front of your load, to keep wind, rain and debris from getting into the tarps and getting something wet, or having "parachute tarps". (parachute tarps are when you get wind under the tarp while your driving, and the tarps bulge out like a parachute full of air, something you dont want)
A "boxed end" is when the sides of the flap are sewn to the front of the tarp, with the front flap edges, sewn to the side drop front edges. Imagine it like a pre sewn fitted sheet for a bed, without the elastic in the bottom. Its good in a way because if you habitually haul the same size load, there is no need to manipulate the front flap to fit the load, and to have to bungee cord it or anything. It is already sewn on, and is water and air tight.
Most lumber tarps, regardless of boxed or flapped, have 3 rows of "D" ring, starting along the bottom edge, and going up every 12" or so, running the length of the tarp, and across the flap or box. This is so you can roll up your excess bottom and still secure the tarp.
Steel tarps, are usually 24' long by 12' wide. Taking the 8' width of the trailer, it means you only have 2' drops on each side. Steel, being heavier, generally will be your shortest load on your trailer, so you may not need 8' of tarp material to roll up. Thats why you would use the short sided tarp. Its good for sheet steel, and rebar. There is no flap on the front. And it usually has 2 two rows of "D" rings, sewn along all 4 edges. They are also good if you have a load of crated material, and they only want a top cover to keep dirt or wetness off of the top.
Smoke tarps are usually 12' by 12', with 2 rows of rings, and are used on the top front of a load to keep exhaust smoke off of the load, or, if you have a load of pipe (steel or PVC) and you dont want the wind whisteling down through the load. They can also be used for very small items that need a cover, but not the wole load.
Bag tarps are just that. Tarps sewn into a bag shape, to go over things like steel coils. (can also be referred to as coil tarps) Imagine putting a stocking cap over your head. Bag tarps are used to go over individual coils, have several rows of rings, and come in various sizes.
Glass tarps are usually a very heavy weave of knitted material (think fishnet, like ladies stockings) that go over crates of glass. They are not water tight, but are more used to keep road debris and junk off of the glass. While I have seen them, I have never used them, so I do not know what shape and size they come in.
Some people have custom made lumber tarps with 4' drops, but have a second set of "clip ons" that is 4' tall, and are however long their main tarp is. That way, if they only need 2' or 4' of drop, they have it with the one tarp, but if they need 8', they clip this piece on, and it extends the drop to 8'. Instead of carrying 2 full sets of tarps, like I did, they have one, with the extenders. Saves ALOT of room and headache, but is rather pricey. On the top edge of the extension, it has clips, and clips onto the lowest edge of "D" rings on the main tarp.