You use Core Data to manage the model (in the model-view-controller sense of the word) objects in your application. Core Data is an object-graph management and persistence framework. Among other things, it:
Allows you to efficiently fetch model objects from a persistent store and save changes back to the store.
Provides an infrastructure for tracking changes to your model objects. It gives you automatic support for undo and redo, and for maintaining reciprocal relationships between objects.
Allows you to maintain disjoint sets of edits of your model objects. Disjoint sets are useful if you want to, for example, allow the user to make edits in one view that may be discarded without affecting data displayed in another view.
Allows you to keep just a subset of your model objects in memory at any given time. This is useful for keeping the memory footprint of your application as low as possible.
Has an infrastructure for data store versioning and migration. This infrastructure lets you easily upgrade an old version of the data file to the current version.
To support this functionality, Core Data uses a schema called a managed object model that describes the entities in your application.