As Wikipedia defines:
Voile is a soft, sheer fabric, usually made of 100% cotton or cotton blends including polyester. The term comes from French, and means veil. Because of its light weight, the fabric is mostly used in soft furnishing. Full-length curtains in hot countries are made with voile and used as window treatments, mosquito nets etc. When used as curtain material they are similar to net curtains.
Voiles are available in a range of patterns and colors (unlike net curtains which are generally white or off-white). Because of their semi-transparent quality, voile curtains are made using specially manufactured heading tape that is less easily noticeable through the fabric.
Voile fabric is also used in dress-making, either in multiple layers or laid over a second material. Voile’s translucency is similar to organdy, which is also used in dress-making.
In order to increase the materials flexibility and resistance to wrinkles, the cotton can be mixed with polyester. A curtain made of 80 percent polyester and 20 percent cotton will be less likely to fade or weaken in the sun, and will also be easier to care for, and possibly machine-washable. Viscose may also be added to the mix, especially if the curtain has a pattern of stripes or dots sewn into it. Some materials used for curtains are often treated with other flame-retardant chemicals for safety reasons.