Backup and Restore vs Export and Import

Here are a couple ways to copy content around in SharePoint:

Backup-SPSite and Restore-SPSite
Export-SPWeb and Import-SPWeb
A couple things to keep in mind:
Backup Restore SPSite is best used on Top sites, so use Backup and Restore SPSite if you are copying an entire site collection and all of it’s sub sites to a new site collection on say a UAT or Production server.
Use Export\Import SPWeb if you are copying a sub site and it’s sub sites to a new location in a different site collection or if you are exporting a particular list or library to later import to a new site.
Make sure when exporting and importing that the site templates of the two sites, match.  In otherwords, if you are exporting from a team site ( webtemplate = STS#0 ) then make sure the target site that you’re importing into is also based on the team site template. This post has good information on exporting and importing sites.