My Technical Definition is Bivy (Derived from bivuac (Military word for Camping)) and sack is a sleeping cover that hopefully is waterproof (imagine sleeping in your rain fly)
Basically it is for folks who like to travel light especially backpackers, bikers and climbers who want to do low impacy camping and be able to sleep just about anywhere.
The Bivy sack is in essence a Mini tent usually without poles but sometimes has 1 pole to hold a bug screen, and it is only big enough to hold a single person and their sleeping bag. Bivy Sacks and/or hammocks are generally a sign of a minimalist leave no trace camper.
To get a good idea of what is marketed as Bivy Sacks I would check out EBay or wikipedia
http://en.wikipedia.org/wiki/Bivy_sack or you can always google it.