Typically, the SAP System landscape consists of three Segregated environments: development, quality assurance and production system.
Development and configuration are first done in development system and then moved to Quality Assurance system. In quality system the changes are tested and once it meets the requirements it is approved. Only when the changes are approved by the testing or functional team, that it is moved to production. If any error is found in testing or functional consultant in quality system, then the changes are again done in development system and then moved to Quality system. After final testing and approval, the final changes are moved to Production system which has actual business data and where actual business activity takes place. Note that customization or development is not allowed in QTST and PROD systems.
You can split a SAP system into multiple clients, each client fulfilling a purpose. For example in the development system, you can have a client for development and another client for customization.
client/system to another is determined by transport layer. The transport layer and transportation of objects is a feature of CTS Central Transport System. The three systems are connected to each other through network for transporting objects as per transport layer specification. This is called three-system landscape.