Clients are essentially self-contained business entities or units within each SAP system; using a web browser or one of SAP's special user interfaces, you log in to a client in SAP to actually access and use the system. Each system—SAP ERP, CRM, SCM, and so on—has a unique system-specific client you log in to. Contemporary organizations thus have multiple production clients (one production client per SAP component). And each component contains several nonproduction clients, as well. These are used to develop and test the business functionality that will one day be put into the production client and handed over to the company's end users. A client has its own separate master records and own set of "tables" (which we cover in detail in Hour 3, "SAP Technology Basics"). The best way to grasp this might be to think about a really large company like ExxonMobil, General Motors, or Honeywell. Within each of these large multinational organizations, for example, you might have three or more other companies or business units. Each SAP client might be tied to a different business unit; really big companies might have two or even three production clients for a single SAP component like ERP. For example, the company might structure its clients around discrete business groups (Chevrolet, Cadillac, and GMC) or by geography (Americas, Europe, and Asia). In this way, a Chevrolet business user might log in to the Chevy client to do her work, whereas business users over at Cadillac log in to the Caddy client on the same SAP system and do their work. In the end, the results can be easily rolled up so that the multinational organization as a whole can easily report on its cross-company financial status, inventory levels, and so on.

Reference: http://www.informit.com/articles/article.aspx?p=1716287&seqNum=3