Overwatch was down last night thanks to a DDOS attack against Blizzard's "network providers," the developer wrote on Twitter.
Blizzard reported issues on its servers just before 11pm CT on July 8—a problem that affected not only Overwatch but many of Blizzard's other games too. Overwatch players reported being kicked from their matches. The DDOS attack lasted until the morning of July 9, according to Blizzard.
A post on the Blizzard customer service Twitter account suggested the DDOS attacks were over at 6am CT. "The DDOS attacks against network providers that we were monitoring have ended," Blizzard wrote. Overwatch developer Bill Warnecke also confirmed the server problems on Reddit. "We're aware of a major service issue now affecting all Blizzard titles," Warnecke wrote. "Sorry for the hassle."
Overwatch players are still upset; many lost Overwatch's competitive points after being disconnected in an ongoing match. Players having problems across the developer's suite of games should contact Blizzard support.
But what is a DDOS attack? DDOS is an abbreviation for a "distributed denial of service." DDOS attacks flood a service with traffic, often enough to overload it and shut it down for a while. Systems get overloaded, and folks looking to legitimately use a site or service are denied. The Wikipedia page for DDOS likens it to "a group of people crowding the entry door of a shop, making it hard for legitimate customers to enter," therefore disrupting trade of services.