Inversion of Control is a very broad principle that can be applied not only to the problem of application extensibility. In fact, in more general terms it can be said that by implementing some form of IoC, instead of the custom code controlling the infrastructure, the infrastructure controls the custom code. With IoC, the various components of an application trade off their power of controlling the flow in exchange for an improved level of decoupling. This is also known as the Hollywood principle or "don't call us, we'll call you".
