Mixed Drinks vs Cocktails

A Mixed Drink is an alcoholic beverage of two or more ingredients. Typically, a base spirit is combined with other distilled spirits, mixers, sweetners, juices and water.

The term Mixed Drink can also refer more specifically to tall or short drinks with a spirit and mixer served over ice, such as the Gin & Tonic and Screwdriver.

A Cocktail is a type of Mixed Drink. The official definition of a Cocktail according to the modern Merriam-Webster Dictionary is "an iced drink of wine or distilled liquor mixed with flavoring ingredients." That's a pretty broad definition, but reflects the modern practice of referring to almost any mixed drink as a Cocktail. The first published definition of the Cocktail appeared in an editorial response in The Balance and Columbian Repository of 1806. This read: "Cocktail is a stimulating liquor, composed of spirits of any kind, sugar, water and bitters." It is this definition of ingredients that still refers to the "ideal cocktail."

http://cocktails.about.com/od/history/a/cocktail_dfntn.htm
http://cocktails.about.com/od/cocktailspeak/g/mxddrnk_spk.htm