A food processor is a kitchen appliance that I once heard described as an essential culinary tool, along with a sharp knife and a pair of tongs. I had my doubts about the utility of each of these until I had obtained them, and I must say that I use them all, frequently, and with great success. A food processor can chop, dice, slice, shred, grind and puree almost any food quickly and easily.
A food processor consists of a plastic work bowl that sits on a motorized drive shaft; its cover has a feed tube through which foods can be added to the bowl. Most come with a set of standard attachments, which generally include an S-shaped chopping blade and several disks for slicing and shredding. Ideally, these should be made from metal, not plastic, as they will stay sharper longer, though some models have both metal and plastic blades and disks. Variations among food processors hinge on the size of the motor and bowl - obviously bigger more powerful machines can more easily handle bigger jobs; the size of the feed tube - my ideal food processor would have a tube big enough for a whole onion or potato; and attachments - some come with juicers, pasta makers, julienne disks, dough kneaders, and even mixers. The food processor body should be fairly heavy, so that it can do its job without bouncing all over the counter.
Originally all I had was a blender, which I used to make smoothies, pureed soups, and pesto. If I had a blender, why would I want a food processor?
A blender makes a better smoothie or pureed soup than a food processor because it blends liquids and even chunks of ice more smoothly; a food processor will leave small chunks of ice and fruit in the finished product. But this same quality means that a food processor makes better pesto or olive paste than a blender can; the food processor leaves very small chunks in the food which give it a pleasing texture. And where a blender requires sufficient liquid to function, a food processor can do its work with no liquid at all.
A food processor, then, is ideal for making pie crusts; add the flour and fat to the food processor, break up the fat with five one-second pulses, then turn in to a bowl and add the liquid. (Most food processor have an "on" and a "pulse" button; the latter will only keep the motor on as long as you hold the button down. This is so you can "pulse" to combine: use short, controlled bursts of power to chop and combine just as much as you need. Continuous "on", on the other hand, can overcombine. Most modern recipe books will give you exact directions for how long, and how many, pulses you need to use.) I use my food processor to slice potatoes and onions for scalloped potatoes, to make topping for an apple crisp (you can slice the apples up with the processor too), to shred mountains of cheese in a second, to make pizza dough and pastry, and for countless other tasks.
A food processor can be an expensive investment, and it might seem like a needless extravagance, but I've found it an invaluable kitchen tool. I bought mine at a garage sale for $5 from someone who didn't understand how useful it can be, so if you keep your eyes peeled, you might get one cheap too. I think you'll be glad you did. | 
