The massive 'denial of service' attack targeted .cn domains, including the popular Sina Weibo microblog site.
With a 'denial of service' attack of this scale usually bot-nets are used. These are comprised of possibly hundreds or thousands of computers. These computers are usually inadvertently infected with malware and other programs that cause them to take commands from the person running the bot-net. In this case the bot-net sent out thousands of request simultaneously to the domain servers which resulted in slow, or completely unreachable sites.
The attack was two-stroke with the first attack happening at 2am and a subsequent attack occurring at 4am. The second wave of attacks was 'long-lasting and large-scale' according to state media. The attack caused websites using the .cn domain to either resolve slowly, or not at all, resulting in many Chinese users not being able to reach the internet at all.
There is no clear motive or suspects currently and no one has taken responsibility for the attack. You can read more on this story on CNN.