Pasted from http://blah.winsmarts.com/2006-12-Backup-Restore_a_site_collection_in_SharePoint_2007.aspx

The best way to backup a SharePoint site is –

Stsadm –o backup –url <SiteURL> -filename <FileName>

For example: Stsadm –0 backup –url http://sharepoint:100 –filename c:\sharepoint100.bak

To restore the site, use

Stsadm –o restore –url <restoretoURL> -filename <filetorestorefrom>

For example: Stsadm –0 restore –url http://sharepointNEW:100 –filename c:\sharepoint100.bak