Generally, the term component denotes an object that is reusable and can interact with other objects in the context of a framework. The term control, on the other hand, denotes an object that is sort of a specialized component. The main trait that differentiates components and controls is the user interface. Components are non-UI objects; controls are primarily UI-based objects. In the Microsoft AJAX library, the root component class is Sys.Component. The root class for controls is named Control and, guess what, lives in the Sys.UI namespace. In the Microsoft AJAX library, Sys.UI.Control derives from Sys.Component.