Allan MacGregor has a recent post to his site looking at another design pattern (previously he discussed dependency injection), this time it's the use of factories.

The factory pattern is one of the most commonly used Design Patterns, it is useful when we need to choose between several interchangeable classes at runtime. By using factories we are also separating the creation of an object from the actual implementation. We can define a factory as a simple design pattern that give us a convenient way to instantiate objects. A factory is capable of creating different types of objects without necessarily knowing what type of object is actually being created.

He talks some about the problems that factories try to solve and some of the common implementations - simple, abstract and the factory method. He also covers some times when it's useful to use the pattern (including the creation of complex objects). The rest of the post includes sample code showing the creation of a Product class, creating them manually and the refactor over to a factory created method, making it more flexible for products in the future.