There is a detailed article on tents.
A bivouac sack (also known as a bivy, bivvy, or bivi) is an small, lightweight, waterproof shelter (like a waterproof jacket for your sleeping bag). It is an alternative to traditional tent systems and hammocks. It is used by nomads as a simple accommodation system that can be used in any environment, without complicated setup.
Advantages - A bivvy bag is a thin, waterproof bag. You use it over the top of your sleeping bag. - A bivvy bag is often cheaper than a good waterproof tent. - It’s a lot smaller than a tent and light to carry. - It’s very discreet and allows you to sleep on tiny patches of flat ground, away from prying eyes. Do not needs to see you set-up, and makes for a quick get away in the morning - You are not cocooned from the environment as you are in a tent. - It feels more exciting and more wild than a tent.
Disadvantages - If it rains you will have a less pleasant night than if you were in a tent. - Depending on the quality of the bivy, there might be some condensation on your sleeping bag.
For further information of bivy bags, hints on setting one up and choosing the eight one, there are many websites out there; including http://www.alastairhumphreys.com.
- For information about how to make a "Do It Yourself" camp stove, see the DIY equipment section.