A web application is a computer program that utilizes web browsers and web technology to perform tasks over the Internet. When your web technology is ZEN then it will called ZEN Application. A Zen Application can be comprises of single or multiple ZEN Pages.
When you want to interact with Client Components like text box value / its style etc, you should use Client Method.
When you want to interact with Server Component like saving the value in GLOBAL, you should use Server Method.
For better understanding you should understand Two Tier Architecture