What is a collection?
Backbone collections are simply an ordered set of models. Such that it can be used in situations such as;
Model: Student, Collection: ClassStudents
Model: Todo Item, Collection: Todo List
Model: Animal, Collection: Zoo
Typically your collection will only use one type of model but models themselves are not limited to a type of collection;
Model: Student, Collection: Gym Class
Model: Student, Collection: Art Class
Model: Student, Collection: English Class
Here is a generic Model/Collection example.