The MSG structure has the following form:
typedef struct tagMSG {     // msg  
    HWND   hwnd;      
    UINT   message;
    WPARAM wParam;
    LPARAM lParam;
    DWORD  time;
    POINT  pt;
} MSG;
The MSG structure contains message information from a thread’s message queue.
Members
hwnd
Identifies the window whose window procedure receives the message.
message
Specifies the message number.
wParam
Specifies additional information about the message. The exact meaning depends on the value of the message member.
lParam
Specifies additional information about the message. The exact meaning depends on the value of the message member.
time
Specifies the time at which the message was posted.
pt
Specifies the cursor position, in screen coordinates, when the message was posted.