Ruby constants and variables

foobar 

A variable whose name begins with a lowercase letter (a-z) or underscore (_) is a local variable.

@foobar
A variable whose name begins with '@' is an instance variable of self.

@@foobar

A class variable is shared by all instances of a class.

$foobar

A variable whose name begins with '$' has a global scope; meaning it can be accessed from anywhere within the program during runtime.

FOOBAR

A variable whose name begins with an uppercase letter (A-Z) is a constant. A constant can be reassigned a value after its initialization, but doing so will generate a warning,

More detailed information about Ruby constants and variables can be found from here:
http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Variables_and_Constants