HKEY_CLASSES_ROOT - Contains all of your file types and OLE information for all your OLE-aware applications.
HKEY_CURRENT_USER - Points to the part of HKEY_USERS appropriate for the current user.
HKEY_LOCAL_MACHINE - Contains information about all of the hardware and software installed on your computer.
Since you can specify multiple hardware configurations, the current hardware configuration is specified in HKEY_CURRENT_CONFIG.
HKEY_USERS - Contains certain preferences (such as colors and control panel settings) for each of the users of the computer.
HKEY_CURRENT_CONFIG - Points to the part of HKEY_LOCAL_MACHINE appropriate for the current hardware configuration.
HKEY_DYN_DATA - This branch points to the part of HKEY_LOCAL_MACHINE, for use with the Plug-&-Play feature.

Now, why do we need the Registry? Well, remember those old .INI files?
They all are now gathered in the registry!
Ofcourse the .INI files are still exist.
It's more orginized, it's faster and it's easier to access!
So if you are used to save your app's settings in messy .INI files, meet the neat usage of the registry:

//Every key you open/create you have to close it, like a file. HKEY hregkey; // Declaring a key pointer for usage. // Here, we create a directory called: "TESTING" long res = RegCreateKeyEx(HKEY_LOCAL_MACHINE, "Software\\TESTING", NULL, NULL, NULL, KEY_WRITE, NULL, &hregkey, &IsNew); if (res == 0) // Creatation was successfull { // Storing a string char str[256]; strcpy(str, "Hello Registry"); // REG_SZ means a string is to be saved RegSetValueEx(hregkey, "String", 0, REG_SZ, (const unsigned char *)str, strlen(str)); // Now a number int num = 128; // REG_DWORD is a long type number RegSetValueEx(hregkey, "Number", 0, REG_DWORD, (const unsigned char *)&num, sizeof(num)); //Don't forget to close the key... RegCloseKey(hregkey); } res = RegOpenKeyEx(HKEY_LOCAL_MACHINE, "Software\\TESTING", 0, KEY_ALL_ACCESS, &hregkey); //Notice we now call RegOPENKeyEx... if (res == 0) { // Let's read the string first int datasize = 256; // The maximum length of temp; RegQueryValueEx(hregkey, "String", NULL, NULL, (unsigned char *)&temp, (unsigned long *)&datasize); // Now the number datasize = sizeof(int); // The maximum length of getnum RegQueryValueEx(hregkey, "Number", NULL, NULL, (unsigned char *)&getnum, (unsigned long *)&datasize); //Now let's delete the Number item RegDeleteValue(hregkey, "Number"); //Now let's delete the directory we created, "TESTING" RegDeleteKey(HKEY_LOCAL_MACHINE, "Software\\TESTING"); // Well we don't need to call RegCloseKey 'cause the key is deleted already.. // But if you don't delete the key don't forget to call it // RegCloseKey(hregkey);

That's it...Very easy and useful.