Variables are used to store data; they are placeholders for concrete values. When writing programs, it's convenient to use variables instead of the actual data, as it's much easier to write pi instead of 3.141592653589793, especially when it happens several times inside your program. The data stored in a variable can be changed after it was initially assigned, hence the name "variable". You can also use variables to store data that is unknown to you while you write the code, such as the result of a later operation.
Using a variable requires two steps. You need to:
Declare the variable
Initialize it, that is, give it a value
To declare a variable, you use the var statement, like this:
var a; var thisIsAVariable; var _and_this_too; var mix12three;
For the names of the variables, you can use any combination of letters, numbers, the underscore character, and the dollar sign. However, you can't start with a number, which means that this is invalid:
var 2three4five;