Salt water is a mixture, but more specifically, it is a solution. A solution is a mixture where one substance is homogeneously or evenly distributed in the other.
The substances in a mixture can be separated using physical means. Salt water is separated into its components by boiling, as the collected steam condenses to form water, and the unaltered salt is left behind. If salt water was a substance, separation would not occur.
Mixtures serve many important uses. Mayonnaise, peanut butter, whipped cream and Jello are all examples of mixtures. An alloy, such as stainless steel, is a mixture of two or more metals.