To begin with, what is a variable? A technical definition would be somewhere along the lines of : A pointer to a location in memory where a temporary value can be stored for use in a program. To the common person that may not make much sense, so I prefer a different definition: A container which holds a value that can be changed. Simply put, a variable is a key word that represents a temporary value. If it helps, you can think of variables as having a variable value. Generally, this value can be changed through instructions given by a program, but some variables are unchangeable depending on the scripting language in use. Many languages have built-in variables which keep a constant value.
Most scripting languages also have some sort of method for deciding the pattern of variable names. In general, variable names may contain letters, numbers, underscores, and dollar signs, but they cannot start with a number. They also cannot contain spaces. While variable names can generally be of any length, if they are too long they become unmanageable.
Many languages are case-sensitive, so it is also a good idea to pay attention to whether your variable names include capital or lowercase letters. In languages where variable names are not case-sensitive, a program may confuse one variable with another if care is not taken during naming. A variable named my_var will look exactly the same as a different variable named My_Var. As a rule of thumb, I never create variables using capital letters. That way, regardless of the language I’m using I will never have conflicting variable names.