If you have a regular SyncSpider account, the Company will be tied to your main account.
They represents the name of your Account, and help you better organize your tasks if you have an Agency account.
If you have Agency account, the agency account will be your main Account. Under it, you may add X number of other Sub-accounts or Sub-Companies.
Sub-companies have the same features and are the same as Companies.
Under every Company, you may have X number of projects.