C++ #define UNICODE

Код:

#ifndef UNICODE
#define UNICODE
#endif 
#include <windows.h>
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
 switch(uMsg) {
  case WM_DESTROY:
   PostQuitMessage(0);
   return 0;
  case WM_PAINT: {
   PAINTSTRUCT ps;
   HDC hdc = BeginPaint(hwnd, &ps);
   FillRect(hdc, &ps.rcPaint, (HBRUSH)(COLOR_WINDOW + 1));
   EndPaint(hwnd, &ps);
   return 0;
  }
 }
 return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE, PWSTR pCmdLine, int nCmdShow) {
 // Register the window class.
 const wchar_t CLASS_NAME[] = L"Sample Window Class";
 WNDCLASS wc = {};
 wc.lpfnWndProc = WindowProc;
 wc.hInstance = hInstance;
 wc.lpszClassName = CLASS_NAME;
 RegisterClass(&wc);
 // Create the window.
 HWND hwnd = CreateWindowEx(
  0,                              // Optional window styles.
  CLASS_NAME,                     // Window class
  L"Learn to Program Windows",    // Window text
  WS_OVERLAPPEDWINDOW,            // Window style
                                // Size and position
  CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
  NULL,       // Parent window    
  NULL,       // Menu
  hInstance,  // Instance handle
  NULL        // Additional application data
 );
 if(hwnd == NULL) {
  return 0;
 }
 ShowWindow(hwnd, nCmdShow);
 // Run the message loop.
 MSG msg = {};
 while (GetMessage(&msg, NULL, 0, 0)) {
  TranslateMessage(&msg);
  DispatchMessage(&msg);
 }
 return 0;
}

Вопросы:

1. Что делают препроцессорные директивы?
2. Что делает L перед string? (L"Sample Window Class";)
3. Можно ли без препроцессорных директив?