If you are in a site's "Admins" group, you will see a link to manage your site's collections. Each site has a default collection which you cannot edit or delete. And if you use "People Pages", you will also see a collection named "People" with sub-collections named for each of your people pages. DO NOT MOVE OR RENAME ANY OF THE PEOPLE COLLECTIONS!
In addition to those default collections, you may add additional collections to hold documents and images that you upload to your site.
NOTE: images are always public, regardless of the privacy settings on the collection to which they belong.
In addition to helping you organize your assets, collections may be used to provide privacy protections for your documents. The privacy options for documents are the same as those for pages. And just like pages, privacy settings cascade: the restrictions apply to items in the collection where you configured it, and to items in collections underneath that one.
To change privacy settings for a collection, click on its name in your list of collections, then click on the "Privacy" button in the upper left hand corner of the edit page. That will give you the following popup. Once you click "Save", your privacy options will immediately start being enforced for all documents in that collection.
Individual documents no longer display information about their privacy protections. It may be helpful to name privacy-enforcing collections after their privacy setting. For example "Password Protected" or "Available to All access.caltech Users".