I have only recently discovered bothy bags. For many years I have been carrying round my Wild Country Goretex bivvy bag in the winter time. I have used it many times for planned bivvies and a few times for unplanned ones.
Now that I take the children into the mountains, I wanted some sort of emergency shelter. Buying five Goretex bivvy bags wasn't really an option, especially as I would have to carry them all. Reading various outdoor web sites, I saw bothy bags mentioned. For the other person in the world who hasn't heard of them, they are essentially a large waterproof tent, with no ground sheet. They are made of very thin material, pack into a small sack and weight 500g or less.
The theory is that you pull the bag over your head and sit on the small "seat" at the bottom. The "seat" is just a bit of material, whose primary purpose is to keep your bum dry and prevent the shelter blowing away. The fabric stops the wind and rain getting in and helps stop body heat leaking out. Although bothy bags can be used by one person, they are most effective when used by two or more people, as this generates a warm micro-climate inside the bag.
The simplest form of bag is just waterproof material. However, you can get bags with windows and pockets, into which you can slot walking poles to help suspend the shelter. Bags are available in various sizes which will accommodate groups from two to twenty.
Since there are five of us, I ended up buying a bag suitable for four, reckoning that I could fit two adults and three children inside. You don't need to wait for an emergency to use your bothy bag, they make great lunch shelters. I recently used mine on Moel Siabod in freezing rain, and a high wind. I was warm and comfortable and I wished that I had bought a bothy shelter years ago!
The shelter that I bought was a Terra Nova Bothy 4. This has a couple of windows and slots for trekking poles. A nice touch is that the stuff sack is permanently attached to the bag, so it won't blow away or get lost.