请问RegisterClass具体做了些什么？

请参考MSDN：
RegisterClass
The RegisterClass function registers a window class for subsequent use in calls to the CreateWindow or CreateWindowEx function.

Remarks
If you register the window class by using RegisterClassA, the application tells the system that the windows of the created class expect messages with text or character parameters to use the ANSI character set; if you register it by using RegisterClassW, the application requests that the system pass text parameters of messages as Unicode. The IsWindowUnicode function enables applications to query the nature of each window. For more information on ANSI and Unicode functions in the Win32 API, seeFunction Prototypes.

All window classes that an application registers are unregistered when it terminates.