Great question, especially since we use these terms interchangeably in some places.
Early access:
Dynamics 365 and Power Platform ship product updates in two release waves. Wave 1 is from Apr - Sep and Wave 2 is from Oct - March. All the capabilities that are automatically enabled for end users only get shipped at the start of the wave. i.e. twice a year in April and October. For all such capabilities, we provide a two month early access period (starting Feb and Aug) for admins to try them in a non-production environment first and get ready before they are enabled for end users. Admins can use this time to update any end-user facing training or documentation. These capabilities in early access are production ready, which means admins can try them in a sandbox and enable them in their production environments earlier than April and October too, as per their convenience. You can read more about early access FAQs here: docs.microsoft.com/.../release-schedule
The updates shipped in the other months (other than April and October) include new capabilities that admins can choose when to enable for their users.
Public preview:
These capabilities are pre-release features primarily provided early to seek early customer feedback. They are not meant for production use and you can find more details about terms of use here: docs.microsoft.com/.../supp-dynamics365-preview
If you try out any feature in public preview, you should wait until it is generally available (GA) before you roll it out to your users in production.
