A very strong, fairly flat, rectangular plastic bag filled with nutrient rich soil, suitable for any number of different things; the grow bag is the perfect possession for those who don't have a garden, or for those who have a garden that is too overgrown to actually grow anything in.
Usage is extremely easy: simply purchase a grow bag in one of a vast number of sizes available, place in desired location, preferably outdoors and in a sunny spot - that is, unless the plants you intend to grow require a shady area. The grow bag will probably already have three or four squares marked out on the top of it, simply take a knife and cut out these plastic squares, leaving four neat holes where your plants/seeds can be placed. Just transfer your plant into the soil that is now visible through one of the holes, or gently push two or three seeds into the soil in each hole. After this a liberal amount of water should be placed in each hole, but be warned: grow bags have a tendency to leak!