Account: An Account is the object that contains your contact database, your assets, your campaigns, reports, etc. An Account is tied to a single website domain.. Employees of the Account can have Account Logins to access ActiveDEMAND.
For a given Account, ActiveDEMAND has a contact database. In the database are Contacts and Organizations.
Contact: A Contact is the ActiveDEMAND object that contains the meta data, the activity history, etc for a single person with one or more email addresses and/or phone numbers.
Organization: The ActiveDEMAND object that groups Contacts with the relationship Employer::Employee. A Contact is employed by the Organization.
For example, a Contact Bob Smith who works for Acme Inc. Bob is the Contact, Acme is the Organization. If you have a form on your website, you can have a name field, an email address field, and a company field (among others). If Bob fills out the form:
A Contact would be created in your Account, and an Organization (Acme) would be created. The Organization would have one employee Bob.