All Apex programs run on-demand exclusively on force.com platform.
First, the application server compiles the apex code into an abstract set of instructions that can be understood by apex runtime interpreter.
After the compilation, the compiled code is stored to metadata.
And now, when the end users initiate the execution of apex by clicking the button or visual force page, the application servers retrieve the compiled instructions from the metadata and forward them to runtime interpreter before returning the result.
