A reusable shopping bag, sometimes called bag for life is a type of shopping bag which can be reused several times: this is an alternative of single use paper or plastic bags. It is often made from fabric such as canvas, woven synthetic fibers, or a thick plastic that is more durable than disposable plastic bags, allowing multiple use.

Reusable shopping bags are a kind of carrier bags, which are available for sale in supermarkets and apparel shops. Reusable shopping bags require more energy to produce than common plastic shopping bags. One reusable bag requires the same amount of energy as an estimated 28 traditional plastic shopping bags or eight paper bags. An unpublished report from the UK's Environment Agency found that when compared to a traditional plastic bag, a canvas or cotton reusable bag would have to be reused a total of 171 times to offset the higher carbon emissions. The same study found however that the average cotton bag is used only 51 times before being thrown away.

Source: http://en.wikipedia.org/wiki/Reusable_shopping_bag