Anybody who's done any school algebra will probably know what a variable is--it's just a named piece of data.
In algebra the data is usually a number, but in E it can be all sorts of things (e.g., a string).
The manipulation of data like the addition of two numbers is known as an expression.
The result of an expression can be used to build bigger expressions.
For instance, 1+2 is an expression, and so is 6-(1+2).
The good thing is you can use variables in place of data in expressions, so if x represents the number 1 and y represents 5, then the expression y-x represents the number 4.
In the next two sections we'll look at what kind of variables you can define and what the different sorts of expressions are.
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