What is the difference between a publication list, a suppression list, and a subscriber list?

Subscriber list (standard list): A basic collection of subscribers used as a send audience. Subscribers can be added manually, via import, or via a form. Supports basic segmentation.
Publication list: A special list type used for preference centre management. Represents an email topic or category (e.g., "Weekly Newsletter", "Product Updates"). Subscribers can opt out of individual publication lists without globally unsubscribing. Used with Commercial send classification — a subscriber unsubscribed from a publication list will not receive emails associated with that list but will still receive emails from other publication lists.
Suppression list: A list of subscribers who should never receive a specific email or any emails. Can be applied at the BU level or at the individual send level. Suppression lists are checked at send time — even if an address appears in the send audience, it is excluded if on the suppression list. Used for legal holds, competitor suppression, and do-not-contact lists.