By default the guest checkout in Magento is enabled and visitors can place an order without registering to the website. Some websites require mandatory login for placing orders, and this default feature should be turned off to disallow guest checkout.

To disable guest checkout, navigate to:

System > Configuration > Sales section > Checkout > Checkout Options
Set Allow Guest Checkout to No

This will now disable any guest checkout in your site.
To enable guest checkout, simply set the above dropdown option to Yes.