Companies are helpful to store the data for you customers. You will also need them to assign projects to a customer to get an overview whom you are working for.
In the company you can enter all the relevant information for it. You can define the contact information, the industry and your individual created company type.
Here you can find more information about how to create a company.
With the help of the company types you can classify them. For example, you can differentiate them by customer, prospective customer or supplier. So, you can show your relation to the company. You can create your personal company type, so you have no limits by that.
Here you can find more about how to create company types
Furthermore, you can define different number schemes. That means, that every new company gets his own individual number. When you want to know how to create a number schemea, you have to look up here.