Contextual scopes are a really useful way of performing searches that are restricted to a specific site or list. The “This Site: [Site Name]”, “This List: [List Name]” are the dead giveaways for a contextual scope (I even think you can get a folder context too “This Folder” but it alludes me most of the time). What’s better is contextual scopes are auto-magically created and managed by SharePoint for you so you should pretty much just use them in my opinion.
You may come across a problem where SharePoint search works fine except for your contextual scopes. Your global search scopes all return results but for some strange reason the contextual scopes are empty…
I came across this problem a couple of times recently and the fix is really pretty simple – check your alternate access mapping (AAM) settings and make sure the host header that is specified in your default zone is the same url you have used in your search content source. Normally SharePoint kindly creates the entry in the content source whenever you create a web application but if you have changed around any AAM settings and these two things don’t match then your contextual results will be empty. Case Closed!
Nick Hadlee's Blog on SharePoint and Other Occasional Rants... 