1、An assignment statement creates a new variable and gives it a value:
Variable names can be as long as you like. They can contain both letters and numbers, but they can’t begin with a number. It is legal to use uppercase letters, but it is conventional to use only lower case for variables names.
The underscore character, , can appear in a name. It is often used in names with multiple words。
2、An expression is a combination of values, variables, and operators. A value all by itself is considered an expression
3、A statement is a unit of code that has an effect, like creating a variable or displaying a value.
When you type a statement, the interpreter executes it, which means that it does whatever the statement says. In general, statements don’t have values.
原文：http://www.cnblogs.com/zxpo/p/5374637.html