How Does Apex Work? Architecture Diagram

All Apex runs entirely on-demand on the Force.com platform, as shown in the following architecture diagram:

1. Developer Action: 

 When a developer saves Apex code, the platform application server first compiles the code into a set of instructions that can be understood by the Apex runtime interpreter, and then saves those instructions as metadata.

 2. End User Action: 

 When an end-user clicking a button or accessing a Visualforce page, the platform application server retrieves the compiled instructions from the metadata and sends them through the runtime interpreter before returning the result.