How Does Apex Work?

All Apex runs entirely on the Force.com platform, as shown in the following picture of diagram:
Apex is compiled, stored, and runs entirely on the Force.com platform

Following are two sequence of flow of actions when the developer saves the code and when an end user perform some action which invokes the Apex code:

-When a developer writes and saves code to the platform, the platform application server first compiles the code into an abstract set of instructions that can be understood by the Apex runtime interpreter, and then saves those instructions as metadata.
-When an end-user triggers the execution of Apex, could be 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. The end-user observes no differences in execution time from standard platform requests.

Note: Apex does not support some features which a general programming language supports. The following example are some features that Apex does not support:
-Render elements in the user interface other than error messages.
-Customization of standard force.com provided functionality.
-It is not possible to prevent the standard functionality execution.
-Creation of multiple threads.
-Creation of temporary file.