The Sencha Class System is built on top of processors, which are divided into two groups: pre-processors and post-processors. Although the term "pre-processors" may sound complex, class pre-processors are much simpler than you may think. They are simply a set of "hooks" that run before that class is actually created and ready to be used. Extend, mixins, config, requires, statics are examples of pre-processors.
Similarly, class "post-processors" are hooks that execute as soon as the class is ready. Singleton, alias are example of post-processors.
Please visit edspencer.net for detailed information on class processors.