Technically speaking Ruby does not need interfaces since it is dynamically typed. Any argument list that at least satisfies the arity condition can be handed into a method.
Since people appreciate the usage of interfaces and even more Design by Contract (DbC) some mechanics were developed that mimic interfaces and certain aspects of DbC. I list them in order of increasing complexity.
(1) Document which methods a method argument must implement or its expected type.
(2) Define a module containing the methods that all raise an exception like this:
def bar(a,b) raise "bar(a,b) must be overridden"; end
def bar(a,b) a+b; end
(3) Create a framework (like the one Paul Brannan suggested here ) for defining interface methods and for checking that a particular instance implements these methods.
(4) Try to support even more of DbC by dealing with preconditions and postconditions like I tried in .
Recently announced 0.1 version of Ruby Design by Contract and Java style interfaces for ruby.