A "Collection" in Aurora, is a kind of PHP array wrapper to enforce the type of the objects within the collection.
A Collection can be easily defined by extending the Aurora_Collection class.
The Aurora_Collection class decorates the PHP array with methods that enforce type checking.
// class to act like array but only accepts objects of type Model_User
class Collection_User extends Aurora_Collection
{
}
Once you have defined the above Collection_User, you have defined a strictly typed Collection to accept objects of type Model_User.