What's the difference between GtkApplication and gtk_init?

I am now learning to use GTK+3.0 with C in Linux. After reading some tutorials and sample code, I have some questions regarding how to initialize an application.

Here are two versions of code I have seen.

#include <gtk/gtk.h>

static void
activate (GtkApplication* app,
          gpointer        user_data)
{
  GtkWidget *window;

  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Window");
  gtk_window_set_default_size (GTK_WINDOW (window), 200, 200);
  gtk_widget_show_all (window);
}

int
main (int    argc,
      char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}

This code used gtk_application_new() to init a GtkApplication and g_application_run() to start it.

This is the second one.

#include <gtk/gtk.h>

int main(int argc,char *argv[])
{
  GtkWidget *window;
  gtk_init(&argc,&argv);

  window=gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_window_set_title(GTK_WINDOW(window),"helloworld");
  gtk_widget_show(window);
  gtk_main();

  return 0;
}

This code used gtk_init() to init the application and gtk_main() to run it.

However, I can't figure out the difference between them as the running result seems the same.