When you are selecting a hammock, particularly for outdoor use, the materials play a significant role in comfort and durability.
The original hammocks were made from tree fibers and supported by woven branches.
But a lot has changed since then.
Unless you are on a Discovery Channel episode let's leave the bark of trees for another time, and get to business on what is best for your modern day hammock.

• Cotton is almost universally agreed to be the most comfortable of the fibers in general use in hammock making.
  It is strong, soft, breathable, and just plain giving.
  In Mayan and Nicaraguan hammocks, cotton is used in the form of string or cords to make comfortable open weave hammocks.
  In Brazilian hammocks it is cross woven into canvas.
  In North American hammocks it generally manifests as cotton rope.
  Although it is the most comfortable fiber, it is an organic material, hence subject to mildew and mold.
  Therefore, if you are going to use it outside, you will need to take some care to either keep it dry, or to let it dry out after a rain or lawn sprinkler episode.
  
• Polyester is a manmade fiber used in a wide range of hammocks.
  Like cotton, it is generally soft and comfortable, and very strong.
  Being non organic in origin it is not subject to mildew, and is often a great choice for the outdoors in wetter condition.
  It tends to stiffen after a few seasons in the UV radiation of old Sol.
  Polyester is found in some Nicaraguan hammocks, and, increasingly, in hammocks made in China.
  
• Polypropylene is surprisingly common in the Mexican style of hammock where it is referred to as nylon.
  It is extremely strong, excellent for jungle use and humid climates.
  It won't mildew, and is long lasting in the shade.
  Under high UV it can deteriorate quickly.
  It forms a harder, stiffer bed than cotton or polyester.
  
• Nylon, true nylon, is a superb hammock material for the great outdoors.
  It is also hard to find, and quite expensive.
  It combines the strengths of polypropylene with a strong disregard for UV radiation.
  It is principally found in high end Mayan hammocks in the support arm structure.
  There are a small number of Mayan hammocks made entirely of Nylon.
  If you are shopping for a Mayan hammock, make sure you know whether the "nylon" is nylon or "polypro".
  
• Duracord is a popular manmade fiber, increasingly found in North American hammocks.
  It is soft to the touch, and very strong and durable (hence the name).
  As it is generally found in the rope and bar style of hammock, its true comfort value is hard to asses.
  But if a rope hammock is your choice, Duracord is a good choice for the bed.
  

Since threads make the bed, pick materials that work best for your blend of comfort, wear, and longevity.