As you may know, in a default Joomla installation, new users can register for your site and then get immediate approval as 'registered users.' Or, you can set it so Joomla sends the new user an email with a "click on this link to confirm" in order to make sure that they have entered a valid email address.
Sometimes, you want a bit more than that, and you want an administrator to approve the registration first. In the past, you would need to install an extension like Community Builder in order to get that ability but now it is much easier with several recently released extensions.
First of all, there is "Registration Approval by Admin." This free plug-in adds one more step to the new user registration process. After the new user has registered and confirmed their email address, Joomla sends the admin an email saying that a new user is requesting approval. You are given a link to click that will confirm the registration. If you do that, Joomla activates the account and sends the new user an email notifying them that they have been approved. It's quick, easy, and adds a very useful layer of spam protection to your site.
The second extension to consider is the "Multilingual Registration Approval." Don't let the word "multilingual" confuse you. The key feature here is "Registration Approval."
It works basically the same was as the extension listed above, except it adds support for JoomFish enabled multi-lingual sites.
Either of these is a good thing to consider for your site if you want admin approval added. I've got a few sites where this is going right away.