[Rails] ActiveRecord::Base

Good question!

Using :: you have access to the class inside the module. Module can nest
another modules and so on. :: its a namespace resolution operator.

Call ::String.new inside some module back you to top-level namespace
(outside the module)

Here is examples

class String
  def initialize
    puts "im string outside module"
  end
end

module ActiveRecord
   module AnotherModule
   end

   class String
    def initialize
      puts "im string inside module"
    end
   end

   class Base
    def initialize
     ::String.new
     String.new
    end
   end
end

Here are results from IRB:

1.9.3-p392 :021 > ActiveRecord::Base.new
im string outside module
im string inside module
  => #<ActiveRecord::Base:0xa024fb8>
1.9.3-p392 :022 > ActiveRecord::String.new
im string inside module
  => #<ActiveRecord::String:0xa023168>

::String means explicit root namespace.

So if you have something like
module MyNamespace
   class String
      ...
   end
end

MyNamespace::String will be your own class, while ::String will still be
the original string class defined by Ruby. If you just type String, it'll
find the closest matching namespace, so if you call String.new from within
MyNamespace, it'll return MyNamespace::String, while if you call String.new
from outside MyNamespace, it'll call the default Ruby string. Writing
::String means you explicitly want the default Ruby string, even if you
call it from inside MyNamespace.