Items in the catalogue are classified by collection. A collection is typically the complete set of material originating from a particular individual or body, e.g. the Stephen Buckland Collection is the set of notebooks, correspondence, photographs etc created or collected by Stephen Buckland.
Selecting Collection from the list of 'Browse by' options on the home page will take you to the list of collections.
For some collections we have written introductory webpages exploring the contents of the collection. See highlighted collections
Collections are structured into hierarchies, a bit like family trees. Each level in the hierarchy has an entry in the catalogue - the collection has an entry, each series has an entry, each item in each series etc:
You can see where a particular entry falls in the hierarchy of its collection by looking at the treeview at the top of the page. For instance, the image below shows the catalogue entry for a photograph by Bernard R Billings. As you can see, this is a part of the series of Kent windmills glass negatives, which itself is part of the Bernard R Billings Collection.