Contacts: the contact was manually added to the contacts database by a user.
Analytics: the contact was created when a third-party form is used on an external page. This usually occurs when a custom event is triggered using the events tool (Enterprise only), or when the visitor fills in a third party form on a Wordpress page with the HubSpot tracking code or the HubSpot Wordpress plugin installed.
Integration: the contact was created via a third-party software that you have integrated with HubSpot. Keep in mind that some integrations will appear under APIkey depending on what information is being passed to HubSpot.
It is also possible for the original source to change if it was updated manually or if a contact visited your site before being added to HubSpot. If these contacts convert on a form or click through an email to a page with the HubSpot tracking code on it, HubSpot will try to match the contact with any previous visits based on cookies. If a match is made, HubSpot will update the Original Source property to reflect the source of the contact's earliest visit to your site. Learn more about why a contact's original source changed.