A variable is a name given to a data value. Variables are identifiers which means when you choose a variable name, it must follow the identifier rules. The rules are:

1. The first character must be an ASCII letter, underscore, or a dollar sign.
2. There should not be a space or tab or newline in a variable
3. The characters after the first can be either number digits or ASCII letters or underscore. Only one underscore between letters or numbers allowed.
4. A variable name cannot be a keyword. Examples of keywords are: int, float, for, function, if, switch, var, while
5. JavaScript is case sensitive which means a name with a capital letter is different from a name with a small letter:
     number is different from NUMBER

Variable Declaration
You can declare (introduce) a variable by writing the var keyword before the variable name. A value can then be assigned later.

Example: 

var numb      // declaration = creating or defining

Variable Declaration and Initialization:
JavaScript allows introducing a variable and immediately assigning a value. This is called initialization. In this case it is not necessary to use the var keyword.

Example: 

numb = 798  // initialization = assigning a value

var numb = 798 // declaration and initiation

Variable Typing:

Variables in JavaScript are untyped i.e. they can hold any data type.

Example:

The same variable is changed to hold a string after it previously held a number

numb = 3
numb = "three"

JavaScript automatically converts from one data type to another when needed.

Variable Scope:

Variable scope is where the variable is defined in the program. This restricts where the variable is used. You may think of scope as "where" the variable is visible. A variable defined inside a function is seen only within the block of that function i.e. inside the { } that follows the function name.

Variables declared outside functions are global variables; i.e. they are visible throughout the script region. All functions can use a global variable. Global variables are discourage because it is not easy to debug as it is not easy to know which function assigned a value to the variables. 
Variables declared inside functions are local variables; i.e. they are defined within that function only. A local variable of a function can only be used inside the function in which it is declared. 

Local variables take precedence over global variables with the same name.

Variable Scope Example