An ArcGIS public account is a free account designed for personal, noncommercial use. This account type is for individuals not associated with an existing ArcGIS Online organization. This general account gives you access to sections of Esri websites that require a login, including free, limited access to ArcGIS Online if you don't have a subscription.
If you have a public account and are invited to join an existing ArcGIS Online organization, you can convert your public account to an organizational account so that all your content moves to the organization.