1) a constant begins with an uppercase letter and it should not be defined inside a method
2) a local must begin with a lowercase letter or the _ underscore sign
3) a global begins with the $ sign; an uninitialized global has the value of "nil" and also produces a warning. can be reffered anywhere in the program
4) instances begin with the @ sign; an uninitialized instance has the value of "nil" and also produces a warning
5) a class variable begins with double @@ and have to be first initialized before being used in a method definition, otherwise you will get an error if you refer to it without initializin