You can store as many companies as you like in your Feng Office installation, but one of them is different from all the others: The so called owner company defines which company (or organisation or group of people) runs a Feng Office installation and therefore has some privileges. All the other companies are called client companies.
The two main differences between these two types of companies are:
You can have as many client companies as you like in a single Feng Office installation. Users of different client companies can collaborate when you assign them to the same workspace.
PLEASE NOTE: Even if users do not share any workspace they still can find out about the existence of all other users from any company. This can be a problem if you have one single Feng Office installation for several clients and these clients should not know of each other, so be careful.1)
A company can have users and contacts assigned to it. In the detail view of a company you can see, edit and create such users and contacts. (Please make sure you understand the difference between users and contacts.)
Workspace permissions are part of a user profile, but there are workspace permissions on the company level as well. To understand these permissions (especially how they relate to the workspace permissions of a single user), please read Understanding user rights.