As a fellow civilian contractor w/ security clearance, I'm with Nadaka on this. The same rules apply where I work. My wife and kids will never see the inside of where I work, because they're not cleared. Hell, we do our own janitorial work because they're not gonna pay to find a janitor with a security clearance.
On the few occasions where they do have to bring in someone for a business reason who's not cleared (visits from HP high-muckity-mucks, an HVAC tech to fix the AC/heating, electrician to fix the lights, fire marshal to inspect the premises, etc), they have to be buzzed in at the door, and then escorted every step of the way. If they're coming my way, my monitor needs to go dark. My desk needs to be devoid of any classified material. When I scribble a note on a piece of paper, it doesn't go in the trashcan, it goes into a locked drop box which is periodically collected and sent directly to an industrial shredder. Bottom line: the government don't play around with this shit when it comes to classified material (whether there's an overuse of classification is an entirely seperate matter).
And I take that responsibility very seriously. I write certain info down and gets into the wrong hands, people's covers are blown and they could wind up dead in very painful ways. Or our networks get hacked. Or even just somebody realizes what we do there, mentions it to a friend of a friend of a friend and then a gun nut militia adds us to their hitlist. Chances of a true security breach? Very low. Consequences? Very high.
Most of the info about security clearance levels is public domain though. DoD is the gold standard, other departments have their own systems which are usually modelled after (and sometimes interchangeable with) DoD.
At the bottom there's SBU: Sensitive But Unclassified. Then Secret, then Top Secret. Then there are additional modifiers that can be tacked on like FOUO (For Official Use Only - a way to tighten down unclassified material to keep it out of the public domain), FYEO (For Your Eyes Only--yes, it's not just the title of a Bond flick), and SCI (Specialized Compartmentalized Information - the jargon for "need to know basis").
And yes, non-citizens are disallowed from holding any security clearances. However, if I'm reading this right, I have to question how much of the Perry staff and Gov. Perry himself have clearances? If not, then someone damn well better have been escorting *them* every step of the way, otherwise they could be charged with trespassing and the contracting firm could lose their contract and security clearances be revoked, especially for the site security officer. Might be a good story there.