Quite a few websites and services (Twitter, SoundCloud, Spotify, etc.) are down today after a distributed denial of service (DDoS) attack on the DNS provider Dyn occurred this morning. Gizmodo has the full list of sites and more information:
Domain Name Servers (DNS) act as the internet’s phone book. Basically, they facilitate your request to go to a certain webpage and make sure you are taken to the right place. If the DNS provider that handles requests for Twitter is down, well, good luck getting to Twitter. Some websites are coming back for some users, but it doesn’t look like the problem is fully resolved.