Self Registration allows any users who come to your portal to log in using their Facebook, Twitter, or Google ID. Self Registration is enabled in the Service Portal Settings.
In many cases, it makes sense to allow users to register themselves. For example, if a product is sold through retail channels, customers will come to the service desk through the organization's website, and register to get support. In such circumstances, there is rarely a need to send an invitation (although it can be done).
In other cases, access to the Support Portal is much more restricted. In a B2B setting, for example, only employees of a client company will be eligible to register as service desk users. In those circumstances, you'll want to Invite Contacts to the Service Portal. That process sends a link that people in your Contacts list can use to complete the registration process.