UPDATE 4:30 pm: Dyn has been fighting attacks all afternoon. This is a coordinated, multi-stage attack. Computer users on the West Coast of the US as well as in Europe are now experiencing issues.
PREVIOUS: Large scale hacker attacks on a company that provides part of the backbone for many internet services crippled major sites Friday.
According to Krebs on Security, a Distributed Denial of Service (DDoS) attack hit Dyn, a New Hampshire company that hosts domain name servers (DNS), a key part of internet infrastructure. (Read here if you want the advanced explanation of what that entails.) The attack started at 7:10 am ET, and just after Dyn reported it contained the attack, it reported a second one was underway: 
(NB: UTC stands for Universal Time Coordinated. It is four hours ahead of the East Coast of the US.) There are reports on the internet (where it’s working) that major sites were down:
Websites currently affected by a #DDoSAttack Twitter SoundCloud Spotify Netflix Reddit Disqus PayPal#DDoS — Rincon Games (@RinconGameNews) October 21, 2016
According to USA Today, the government is looking into the attacks:
White House Press Secretary Josh Earnest said the Department of Homeland Security was “monitoring the situation” but that “at this point I don’t have any information about who may be responsible for this malicious activity.”
The attacks have mostly affected users on the East Coast. You can monitor the latest updates from Dyn at their status site, DynStatus.com.