When you cover a load in the back of your pickup, there are a few factors that will affect your choice of truck tarp. Consider each of the questions below before making a purchase.
Are you covering the load to keep it in the pickup bed or to keep the load dry?
If you need to keep the load dry, you will want to go for either a waterproof tarp (like poly tarps) or a water resistant tarp (like treated canvas tarps). If you choose a water resistant tarp, you should take care that you keep the tarp taut and angled so water won't pool and seep through.
If you are only trying to contain the load, do you need a breathable tarp?
Some loads do best if they are covered with a tarp that can allow air to pass through. There are two basic options for a breathable tarp cover: the aforementioned canvas tarps (which are water resistant if treated) or a mesh tarp. A mesh tarp provides far more breathability than a canvas tarp.
Will you be driving at highway speeds, especially for extended periods?
The faster you intend to drive, and the longer you intend to do so, the higher strength you will need to withstand such speeds. A heavy duty poly truck tarp will do the trick for light duty applications, as will heavier canvas tarps, but for long haul highway use, the best option is vinyl. Regardless of your choice, you need to secure the tarp well, preferably with solid rubber tarp straps. Wind whipping will destroy any tarp eventually.