Albums are what. Circles are who.
An album holds photos. A circle is a named group of people — "grandparents", "cousins", "school friends" — that you share particular albums with. The album stays yours and stays on your storage; the circle just decides who can see it.
Circles are optional. If you've set up a Family Hub with the people you live with, you already have a working archive. Circles are for showing things to people beyond that: the grandparents who want the school play, the friends who were on the holiday.
These pages cover both, plus the practical business of getting people in — invitations, what each kind of access lets someone do, and the list of everyone you're connected to.