While not a KEY criteria for all players making a racquet selection, a racquet's string pattern can be a factor of importance for the more experienced player. Racquet String Patterns are commonly referred to as Open or Dense.
An OPEN string pattern has fewer strings with more space between them. It will help when you want to put spin on the ball because the strings can more easily bite into the ball. Obviously, spin generation relies heavily on the technique of the hitter, so if you don't use topspin form, this is a moot point. In addition, because there are fewer strings to absorb the impact of the ball, hard hitters will wear out strings faster with an Open string pattern.
A DENSE string pattern has more strings spaced closer together. While more difficult to generate spin with, this type of string pattern will provide more durability for frequent string breakers and will typically provide more control.