You might think of a company as a mini database within a database. The Company Detail window includes the same tabs as other views including Notes, History, Activities, Opportunities, and Documents. However, there is one major difference: you can filter the Notes, History, Activities, and Opportunities tabs to show just the information that pertains directly to a company. For example, your Company record might be linked to several contacts; you can view all the notes for all of those contacts on the Notes tab of the Company record. Or, should you be confused by all those notes, you can filter the notes to just those notes that were created for the company itself.
In order to see this feature at work you'll want to make sure you have a Company record that contains several linked Contact records. You'll also want to make sure that you've created notes for most of those Contact records.