The ERP system has the idea of a Partner. What is the relationship to the company, what can you do with a Partner in the Odoo system?
A 'partner' is either a person, an association, an organization (company*). * It is better if you don't use company in this definition, as it has another meaning in Odoo.
A 'company' (the way it is more commonly used) is a set of partners (mainly customers, suppliers and employees), users (logins).
An example of a company would be Odoo S.A, Odoo USA or Odoo Asia/Pacific.
A 'customer' is a partner that purchases products from a company.
An 'employee' is a partner that works for a company.
A 'supplier' is a partner that sells products to a company.
Odoo is implemented by people who work for Companies. People are modelled as partners, as are the individuals and organizations that those people buy from, sell to, employ and do business with.
The concept of Partner models a business entity (company) or a person and is one of the central concepts of the ERP (class res.partner). You can find Partner objects for example in the menus Sales-->Customers (or Suppliers). The ERP relates many models to the partner (e.g. Sales Orders, Invoices, Manufacturing Orders, Projects, Tasks, Analytic Accounting, Payments etc). The company (res.company) has an associated Partner, which stands for the company itself. You can use it in all places in Odoo that expect as data type the Partner object.
I hope this helps.