With Christmas day imminent, you can understand that online shopping websites have seen larger amounts of traffic than usual. Popular sites such as Amazon and Wal-Mart are some great examples of these, but they're also prime targets for DDoS attacks, as demonstrated earlier in the day.
As CNET reported earlier, a DDoS attack was launched on the servers of Neustar, a company that provides DNS services for other websites, under the name UltraDNS. Neustar confirmed the attack on Wednesday afternoon, stating that they managed to get things under control fairly quickly, before too much inconvenience was caused. The biggest issue here is timing; DDoS attacks aren't a rarity around the Internet, though when it affects extremely busy websites days during a massive shopping event, things can get very bad, very quickly. In the case of Neustar, the problem arose from the fact that websites need DNS providers to convert the regular URLs you see commonly around the Internet into the IP addresses which websites use. When a DNS provider becomes overwhelmed with requests for IP addresses, such as in the case of a DDoS attack, it causes problems and prevents people from getting to where they requested to go.
Of course, Amazon and Wal-Mart weren't the only ones affected; dozens of sites were inconvenienced by the attack one way or another. Some became sluggish, whilst others were completely unavailable, as Amazon's S3 and EC2 services were affected. Some customers reported issues with purchases, leading to some deliveries being delayed until Boxing Day, which is obviously a day too late for Christmas. The attack was significant enough to case 'ultradns' to be the top search on Google for a brief period of time, showing the extent to which the attack troubled website technicians.