Shortening is hydrogenated, which, as jennyema has pointed out, is simply the ability to maintain it's solid form at room temperature. this requires hydrolyzation of an oil (read: fat) to achieve this result. if you are trying to get past all hydrogenated oils, and still need shortening for recipes, I suggest lard. It is rendered from pig fat, and is therefore naturally solid at room temperatures, and hasn't had any hydrogen processed into it to make it solid. Personally, I also think it tastes better, as most natural fats tend to, and it won't overpower things like butter can. Finally, butter can be substituted sometimes, but you are adding a liquid as well as a solid, since you have things like whey, and milk solids. as posted earlier, butter is only about 80 percent fat, so if you are using recipes where ratios aren't very tolerant, it's not a good substitute, or if you are making quantities,(where exact ratios are important in baking) you can't get away with subbing butter.