A flex application runs entirely in the client environment.  This provides users with a highly responsive experience similar to traditional desktop applications.  Traditional HTML based applications would require page refreshes or post-backs when requesting additional data or page changes.  With Flex, this is all managed within the compiled application, which is already running in the client environment.  This type of interaction has a great advantage in terms of user experience, CPU utilization and network performance.  With the various Flex API’s, Flex is able to perform server-side communications by making asynchronous requests to a server with a very little amount of logic.