A collection is a group of items, that share the same set of fields. Every item uploaded to your digital repository will be part of a collection - and only one collection. For instance, you could have a "paintings" collections, with fields such as Title, Author, Country, Tecnique, etc and another collection for "films", with Title, Director, Country and Genre.
For each collection you can set a different set of fields and they can share common categories, which means you could browse for items in a specific Country, and get both paintings and films in your results.
Items are the actual content of yout repository. The painting, the film, the book and so on. They belong to a collection and have all the fields configured in the collection it belongs to.
Each field has a set of settings. Is it required? Is it supposed to be unique (an ID number for example)? Does it accept multiple values? What is it Field Type? (TODO: see the complete list of field attributes).
You may have repository-level fields, that will be inherited by all collections of your repository. In the same way, collections inherit fields from their parent collection.
Field types are the objects that represent the types of field that can be used. Examples of Field Types are "Text", "Long text", "Date", "Relationship with another item", etc (TODO: see full list).
Filter types are the different types of interfaces to filter items in a collections based on one specific Field. Examples of Filter Types are "input text", "datepicker", "date range picker", "number range slider", "list of checkboxes", etc.