GlobalID is a way of serializing a model into a string, which can then be used to look it up later, without the caller having to know the class. This is helpful in many cases where you accept different classes of objects, but want to do so through string references.
One example is jobs. We don't want to pass in full model objects to a job queue because the marshalling of this object might well not be safe (given that the model object can hold references to database connections or other assets). So instead we pass a GlobalID that can use used to look-up the model when its time to perform the job.
Another example is a drop-down list of options with Users and Groups. By serializing both models with GlobalID, we can make a receiver that simply takes a GlobalID -- or in the case of data that's being exposed to the world, a SignedGlobalID -- and we can deal easily with objects of both classes.
You can mix in ActiveModel::GlobalIdentification into any model that supports being found with a #find(id) method. This gem will automatically include that module into ActiveRecord::Base, so all records will be able to use the following methods:
person_gid = Person.find(5).global_id # => <#ActiveModel::GlobalID ...
person_gid.to_s # => "Person-5"
ActiveModel::GlobalLocator.locate(person_gid) # => <#Person id:5 ...GlobalID is being developed in a gem, but is intended to be included in Active Model as part of Rails 4.2.
Active Model: GlobalID is released under the MIT license.