A company is any organisation that you have a relationship with, be this commercial or otherwise. Companies are added via the Address Book plug-in and have general contact information.
There are specialisations of company for organisations that you have commercial relationships with. These can store additional information relating to the commercial relationship. The most significant of these are customers and suppliers. A company can be converted into one of these.
A company is a categorised record.
Important: you cannot start projects for companies, only for customers.