If you search for information with Google, you might occasionally find that your results are infiltrated with useless copy-and-paste sites. Fortunately, you can block these from your results if they turn up, provided that you're signed in to a Google account when you're searching.
It's also possible to define a list of known bad sites even before they appear in a search, by going to google.com/reviews/t. Anything you search for will automatically exclude results from those sites mentioned. You also use the same page to unblock sites that you have accidentally blocked (or that you want to include back in, if it cleans up its act).
There's more information in the help page and the blog, which announced it last month. The manual site list is a useful way of populating against known bad sites; note that the block option only gets shown if you first navigate to a site, then return back again; since Google's algorithms expect you to have gone to the site to discover its uselessness.