Fundamental to Mathematica's dynamic interactivity capabilities is a new form of symbolic dynamic language. With a very small number of highly powerful primitives that mix freely with other language constructs, you can write a program in a direct symbolic style, and Mathematica will automatically track dependencies to make controls and output of any kind update dynamically.
Dynamic — display as a dynamically updated settable expression of any kind
DynamicWrapper — display an object that dynamically updates an expression, but does not display that expression
DynamicModule — localize dynamic constructs
With — insert static content into a dynamic object