The Senate hack does not appear to have been a remarkably deep one. The hackers were able to get into some of the public-facing parts of the site but not, evidently, past the firewall that protects the more sensitive parts. The group claiming responsibility for the hack is called Lulz Security or, more commonly, LulzSec.
LulzSec is also thought to be behind some rather high-profile attacks on PBS and Sony. There are some political motivations in play here: the Senate attack may have been a response to the Obama administration's aggressive talk on treating cyber attacks as acts of war. The PBS attack was likely a response to a documentary about Wikileaks. But the tone of LulzSec, both on Twitter and on their own behavior, is cheeky, irreverent and mischievous. Even the name "Lulz" refers to online behavior that gets a laugh.
We talk to Gabriella Coleman, professor of media, culture, and communication at NYU. She says, "What they are so good at doing is enacting 'lulzy' behavior that is at once humorous and terrifying -- it makes you laugh and it makes you cringe at the same time." Laugh because they pull pranks like saying Tupac Shakur is alive on the PBS site. Cringe because whoever LulzSec is, they've managed to pull off several profile hacks in the relatively few weeks since becoming publicly visible.
We also check in with Chester Wisniewski of the security firm Sophos. He says LulzSec walks the line between pranksters and thieves. "Customers of these organizations being exposed through these hacks are sometimes put at genuine risk for identity theft or bank account fraud. So it is a rather serious crime. On the other hand, making these companies aware they need to treat security more seriously is important."