From ReliCodia

MSG is the Windows structure that is posted by Windows in the Message Queue to be retrieved by the program in the Message Loop by calling GetMessage or PeekMessage.

Type Name Description
HWND hwnd Handle to target window
UINT message Message ID
WPARAM wParam Message parameter
LPARAM lParam Message parameter
DWORD time Time message was posted
POINT pt Cursor position (in screen coordinates) when message was posted

After the message has been dispatched using DispatchMessage to the appropriate Window Procedure, the information about the time and position may be retrieved using GetMessageTime and GetMessagePos, respectively.