>I have a web app , which includes some MyForm.Aspx.Cs files on the server. Are these .CS files executed directly when the web app is running. If I change one of the .CS files on the server, should the WEB app refelct this immediately, or is there a complied program running ?

We usually do not store those source code files on the server. In fact, you usually do not require to have to App_Code directly on the server. Simply compile that into a DLL and put the DLL in the Bin directory. So, basically, the DLL will already be pre-compiled. Then, the server will only need to load it into the memory and that takes about 8 to 10 seconds.