A Company sits at the top of the hierarchy where Contacts can be created and Sales Ledgers can be set.
From a Company, you're able to view all Contacts, Opportunities, Problems, Quotes etc. which sit under it.
Selecting one of the pages on the left-hand side, such as Details or Address, will show any records created at any level of the hierarchy under the Company.
Additional records can be created within a Company and this is done by selecting the plus icon on the right-hand side.
Do you have a customer that operates from multiple locations? Consider using Company Groups!