:: Lets you access a constant, module, or class defined inside another class or module. It is used to provide namespaces so that method and class names don't conflict with other classes by different authors.
When you see ActiveRecord::Base in Rails it means that Rails has something like
module ActiveRecord
class Base
end
end
i.e. a class called Base inside a module ActiveRecord which is then referenced as ActiveRecord::Base (you can find this in the Rails source in activerecord-n.n.n/lib/active_record/base.rb)
A common use of :: is to access constants defined in modules e.g.
module Math
PI = 3.141 # ...
end
puts Math::PI
The :: operator does not allow you to bypass visibility of methods marked private or protected.