Scott McCulloch has a nice article "Exporting And Importing DotNetNuke Portals”.
I want to add that exported portal can be used to replace existing portal using Wizard.
It also can be used as a default portal by changing Install\DotNetNuke.install.config file by specifying
<portals> <portal><templatefile>NewPortal.template</templatefile>
It is assumed that NewPortal.template and NewPortal.template.resources files are located in \Portals\_default folder.