Mylar bags, also known as foil bags, are made from a polyester film called BoPET (biaxially-oriented polyethylene terephthalate). They are known for their durability, heat resistance, and ability to block out light, moisture, and oxygen. Mylar bags are commonly used for storing and preserving food, pharmaceuticals, and electronics, as well as for packaging and insulation. Their reflective surface also makes them popular for use in emergency blankets and as a material for decorative purposes.