A Supplier is an entity (company or organization or physical person) that you receive/purchase products from.

A Client is an entity (company or organization or physical person) that you ship/sell products to. 

The supplier must be chosen prior to creating a Purchase Order or a Receive On Consignment document.

Similarly, a client must be chosen prior to creating a Sales Quote, a Sales Order or a Ship On Consignment document.

The Contacts that are defined under a supplier or a client can be viewed and updated by clicking the contacts icon next to the respective supplier or client name.