According to a Twitter post by Cloudflare CEO Matthew Prince, the full volume of the attack exceeded 400 gigabits per second—making it the largest DDoS attack ever recorded.
Some nefarious hacker types are using an attack called Network Time Protocol (NTP) reflection which basically sends bogus packets to an NTP server which in turn makes that NTP server send data to a targeted server. When done on a large scale, that’s a lot of data to be sending and blocks legit access. Cloudflare have handled it well and the worst of the attack seems to be over, but now sets a precedent for future attacks on unprotected servers. Not only is there DNS reflection, now NTP can be used as an attack vehicle too.