There is really only one fiber that is suitable for outdoor carpet: polypropylene, usually referred to as "olefin." A by-product of the petroleum-refining process, polypropylene is dyed while it is still a liquid (solution dyed) so the color is a permanent part of the fiber and thus won't fade or stain.
Some of the artificial-turf products are made with a nylon fiber for its tensile strength. Since these are usually constructed in a cut pile, the stronger fiber is needed for the cut tips of the fiber to stand up to foot traffic.
There are also products called "outdoor sisal" for a more natural, woven-grass look. The "outdoor" part of "outdoor sisal" means that it is really made out of polypropylene, but colored and constructed to look like sisal grass.
Tufted carpet is the least expensive to make, but requires special attention when used outdoors. The fiber tufts are pushed into a primary backing and then a second backing is adhered for stability. However, exposed to the weather, these two backings could separate. The need for a second backing can be eliminated by instead adding a latex coating, making the carpet more durable but harder to roll, ship and install. Tufted carpet can be made in "cut pile" or "loop pile." The latter is preferable for durability as people are then walking on the twisted sides of the fiber instead of the cut tips of the fiber.
Woven carpet is an excellent choice for outdoor carpet construction. Made on a loom, the carpet fibers are interlocked and cannot "run" or be separated. This type can also be made in cut pile or loop pile.
A popular and more durable construction for outdoor carpet is "needle-punched." Like woven carpet, the fibers are interlocked---but with needles---and then put in a latex bath to hold it all together.
Outdoor carpet is never installed over a pad. It is either "direct-glued" or, in case of a small space, can be laid in place and the edges folded under and stapled into place. The most important issue is to get adhesive specifically made for outdoor use. These adhesives are not water soluble and have a chemical added to make it resistant to mold and mildew growth.
At many golf course clubhouses, the carpet for outdoor landings used by the golfers is spike-proof. To achieve this, it has a cut pile of around half an inch high with enough tufts per square inch to give it a fiber density (ounces per cubic yard) of 3200 or more. This construction keeps the golf-shoe spikes from reaching the carpet backing and tearing up the carpet.
"Marine carpet" is just a low profile outdoor carpet used for boats. It frequently has a "rubberized" back and is available in smaller quantities for the do-it-yourself crowd. Be sure to get marine adhesive for the installation.
If the carpet gets something like mud or bird droppings wedged down in the fibers, you may need to get on your knees with a scrub brush; otherwise you can clean it with a garden hose, dish soap and a broom. Just let it air dry. Also, the dye method used in outdoor carpet fibers keep it from staining and even bleach won't discolor it.