Shelter systems fit into one of four broad categories. Tents, tarps, hammocks and bivies.
Undoubtedly the most common form of shelter. Size/shape, free standing ability and weight & cost are the key considerations in selecting a tent. The vast majority of tents come with an inner tent and outer fly sheet – it’s worth checking though. Unlike hammocks, tents need to be placed on relatively level ground, free from rocks, roots and the like.
A tarp is simply a sheet of material which usually has either eyelets or cord to secure it to trees and/or the ground. Some my utilize your walking poles at either end and a few come with poles (this pretty much makes it a tent in my book). Avoiding all the poles significantly reduces the weight, however, now some further skill is required to select an appropriate site and secure your tarp. Most tarps are broadly rectangular with some webbing tape running across what is supposed to be the apex. Tarps are usually completely open at two ends. Tarps are frequently used in conjunction with hammocks.
Hammocks are generally designed to be secured off the ground between two trees. Since the hammock is off the ground, the ground itself does not need to be flat, level or free of debris. The trees however, do need to be roughly the right distance apart and sufficiently sturdy to take the weight. A tarp is typically used over the hammock to keep the elements out. Sleep pads can be used with hammocks but not very effectively. Pads tend to want to move about and typically are not wide enough to insulate the sides. Under quilts are desired. Hammocks make pretty great chairs before bedtime. Hammocks don’t work well for folks who prefer to sleep on their front – sideways and back sleeping only.
Bivies are basically sleeping bag sized tents. Sometimes the bivie is an integrated bag and tent in one unit. Usually the only pole ( if any) is to lift the material off your face. Personally, I feel too constrained in a sleeping bag and prefer a quilt. One day I will try a bivie, but so far I’ve managed without facing that claustrophobic demon.