There is an attack timeline which is useful:
Starting at approximately 7:00 am ET, Dyn began experiencing a DDoS attack. While it’s not uncommon for Dyn’s Network Operations Center (NOC) team to mitigate DDoS attacks, it quickly became clear that this attack was different (more on that later). Approximately two hours later, the NOC team was able to mitigate the attack and restore service to customers.
After restoring service, Dyn experienced a second wave of attacks just before noon ET. This second wave was more global in nature (i.e. not limited to our East Coast POPs), but was mitigated in just over an hour; service was restored at approximately 1:00 pm ET. Again, at no time was there a network-wide outage, though some customers would have seen extended latency delays during that time.
And here is some information about the attack itself (not nearly enough information in my opinion):
At this point we know this was a sophisticated, highly distributed attack involving 10s of millions of IP addresses. We are conducting a thorough root cause and forensic analysis, and will report what we know in a responsible fashion. The nature and source of the attack is under investigation, but it was a sophisticated attack across multiple attack vectors and internet locations. We can confirm, with the help of analysis from Flashpoint and Akamai, that one source of the traffic for the attacks were devices infected by the Mirai botnet. We observed 10s of millions of discrete IP addresses associated with the Mirai botnet that were part of the attack
The other articles about this attack are far more thorough in exploring how the Mirai code works and how it can infect Internet of Things devices and where the attacks originated from.