A bivy is a low-profile, waterproof barrier for your sleeping bag that offers an ultra-lightweight alternative to traditional tents. Originally used by climbers who needed lightweight, compact shelters for multi-day ascents, they are also popular among backcountry soloists and minimalists who want to shave every possible ounce of weight from their load. Bivy sacks are basically slipcovers for your sleeping bag that shield you from the elements and slightly improve insulation. They don't provide much room to sit up or move around in, but for the committed minimalist the weight-saving benefits they provide outweigh the cons.
Structured Bivy Sacks:
Structured bivy sacks, called bivy shelters, have small suspension systems that expand the head area to provide some room to sit up in. This makes the confined space more comfortable.
Most bivy sacks use two different fabrics to balance protection and weight. The bottom fabric is heavier for durability. The upper fabric is usually lighter and features a waterproof/breathable laminate.
The most basic bivy sacks have an opening at the head to let you breathe and allow moisture to escape. More advanced models use mesh panels to keep bugs out.