A Design Pattern essentially consists of a problem in a software design and a solution to the same.  In Design Patterns each pattern is described with its name, the motivation behind the pattern and its applicability.  Understanding the applicability of a design pattern is of utmost importance prior to implementing it.

According to MSDN, "A design pattern is a description of a set of interacting classes that provide a framework for a solution to a generalized problem in a specific context or environment. In other words, a pattern suggests a solution to a particular problem or issue in object-oriented software development. Additionally, patterns take into account design constraints and other factors that limit their applicability to the solution in general. Together, the classes, the communication and interconnections among those classes, and the contextual specifics define a pattern that provides a solution to any problem in object-oriented software design that presents characteristics and requirements matching those addressed by the pattern context."

"A design pattern systematically names, motivates, and explains a general design that addresses a recurring design problem in object-oriented systems. It describes the problem, the solution, when to apply the solution, and its consequences. It also gives implementation hints and examples. The solution is a general arrangement of objects and classes that solve the problem. The solution is customized and implemented to solve the problem in a particular context". -- Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides.