What is a Guest User?
Guest User is the meta-user whose layout is presented as the (unmodifiable) layout of the unauthenticated user. A portal maintainer "logs in as guest" to adjust the "guest layout". Changes to the guest user's layout are reflected in the unauthenticated view on the portal after the portal is next restarted, since the guest layout is aggressively cached.
You can have (in theory anyway) have more than one Guest User and different unauthenticated layouts among which your portal switches (say based on the IP address from which the request originates – an on campus unauthenticated view and an off campus unauthenticated view – but this is an advanced usage and probably doesn't work perfectly since probably no one is actually exercising it.
How do I log in as the Guest User?
If you're using an external authentication mechanism, such as LDAP, then maybe you have an external user named guest with a pasword. But you probably don't. So you need uPortal to provide a local mechanism to authenticate the guest user.
Out of the box, the guest user does not have a local password. There are two basic ways to configure the guest user with a password. One way is with an Ant task (accomplishing this task from the command line.) Another way is via an administrative channel.
Configuring a guest user password via the administrative channel is a two step process. First, create a user with username "guest". Second, set the password for that user.
How does DLM change how I might think about the Guest User?
While you can log in as the Guest User and manage the layout of the unauthenticated view of your portal by changing the guest user's layout under DLM, you don't need to do this. Instead, you can compose the guest layout of DLM Managed Fragments via dlm.xml and DLM Layout Owners whose layouts make up the content of Managed Fragments. Advantages of going this route include the ability to use the same managed fragments for both the unauthenticated view and authenticated views (you might want to be share a Main tab across both contexts, for instance) and the ability to delegate management over pieces of the Guest Layout.
It doesn't make a whole lot of difference which route you go and you can use both guest user layout and managed fragments to compose your unauthenticated user experience, but that's needless complexity.