A Scheme program is composed of expressions and definitions (we cover definitions in Section 3.5). An expression is a syntactic element that has a value.
The act of determining the value associated with an expression is called evaluation. A Scheme interpreter, such as the one provided in DrRacket, is a machine for evaluating Scheme expressions. If you enter an expression into a Scheme interpreter, the interpreter evaluates the expression and displays its value.
Expressions may be primitives. Scheme also provides means of combination for producing complex expressions from simple expressions. The next subsections describe primitive expressions and application expressions.Section 3.6 describes expressions for making procedures and Section 3.7 describes expressions that can be used to make decisions.