I am using vs2010 express, I want to post my game. how do I compile the release version of my code?

Also, don't forget to make sure that the release mode configuration properties of your project are set correctly (e.g., include all of the necessary libraries you also used for debug mode).

Why I can't just go with debug result? I get .exe file and program works...

It works on your system. A debug build (usually) binds to debug libraries which not only will be slower due to run-time checks but rarely be available on a user system (Edit: for late binding). Have a look at what DLLs your exe loads with e.g. Process Explorer. Debug DLLs usually end with a 'd'. (Sure you could provide an installer, but that's not something you wanna do normally).

Why I can't just go with debug result? I get .exe file and program works...

The debug build contains a lot of extra code used for debug purposes (such as bounds checking and other possible errors). The release build is highly optimized and does not contain this extra code, making it run significantly faster than a debug build.