Given the demographic of this site, I thought I'd be insulting everyone's intelligence by offering up a more verbose description. I mean the description is the command itself! And verifying the contents of the URL is as simple as viewing a web page (if anyone can't manage that then I wonder how they're reading that command to begin with :p).

With `curl http://someonewhocares.org/hosts/hosts | sudo tee -a /etc/hosts`, you know that the damage is contained to appending malicious data to /etc/hosts. Furthermore, the use of tee guarentees that you see the data that gets appended. Additionally the potential damage of malicious data in /etc/hosts is minimal if quickly detected and fixed.