Generally, the term "mixed drink" is identical in meaning to "cocktail"- a beverage consisting of at least one alcoholic beverage and at least one other liquid, either alcoholic or non-alcoholic. There are other ingredients which can be included as well, such as spices, fruit, and honey (and very often, ice). Note that generally the term "mixed drink" does NOT refer to a shot (or shooter)--even a shot which is comprised of several mixed ingredients, and refers only to a standard, sippable drink.

Mixed drinks became very popular in the United States during prohibition, due to the low quality of the home-brewed alcohol available at the time. Mixing bootleg alcohol with other liquids helped to hide the foul taste and odor of the liquor. Although most mixed drinks now deal with near-flavorless vodka, there are also many recipes that use gin, whiskey, and rum.

Mixed drinks vary widely in complexity, both in ingredients and technique. Some are as simple as a scotch and soda or a screwdiver, and some have longer lists of ingredients, like the infamous Long Island Iced Tea and its multitude of variations. To make many mixed drinks, specific glasses are preferred.

For making most mixed drinks, it is advisable to have certain equipment handy. One and one-half ounce jiggers are easy to find in any kitchen store, and a strainer is adviseable for anyone who needs to make mixed drinks that chill, but are not served with ice. Shakers of all shapes and sizes are easily obtainable, and I recommend a short paring knife for anyone who needs to fix fruit garnish or add rimmers to their drinks. I would also recommend keeping a bottle of bitters handy, in case they have to make an old-fashioned drink, of which there are several.

The more complex a mixed drink is, the cheaper the alcohol you can get away with using. You shouldn't have a problem using cheap stuff for a Slow Comfortable Screw Up Against the Wall, but you'll notice a big difference between a vodka martini made with Crystal Palace and one made with Grey Goose.

Many mixed drinks can be found at the Everything Bartender, as well as at a multitude of other specialized sites.

Good luck and have fun!
http://en.wikipedia.org/wiki/Mixed_drink
http://en.wikipedia.org/wiki/Prohibition
http://www.webtender.com/
http://www.mixdrinx.com/