I'm currently taking an advanced diagnostics class at Lansing Community College as part of my automotive technology degree. I'm learning how to properly diagnose a problem with anything related to driveability, and the electronic systems on a modern car. Since 1996, cars have gained more and more electronic controls and driver aids. While some of it is designed purely for creature comfort, most of it serves 1 purpose: to reduce the amount of harmful emissions that exit the tailpipe. That;s what it all boils down to. The Oxygen sensors, for example, monitor the amount of oxygen in the exhaust. If they sense too much oxygen, indicating a lean condition, the computer sees that, and commands the injectors to deliver more fuel until the oxygen level in the exhaust returns to its most efficient level.
It does get more complicated from there, but unless you're an automotive technician, an engineer or work at an emissions testing facility, it would do no good to go any further with the specific jobs of each sensor and how it relates to emissions.
When the phrase "Emissions Test" is mentioned among many car enthusiasts, it brings up a sense of hatred or disgust. Why? more than likely, their vehicle has been modified in such a way that would cause it to fail an emissions test, be it by removing the Catalytic converter to improve exhaust flow, or upgrading the turbo to create more intake pressure.
It is possible to meet the emissions standards for any car, modified or not. Many people don't know this, but if a car can pass an emissions test, it's usually running at its most efficient state. Some, however can not, due to either aggressive performance tuning, or a poorly kept-up vehicle.
In states that require emissions testing, if a vehicle does not pass an emissions test, it can't be registered. All faults causing the vehicle to fail must be repaired properly in order for the vehicle to be registered. All vehicles, however, are not subject to emissions testing. Any vehicle used strictly for off-road use (ORV's, Dedicated Race vehicles, and machinery) is not required to be tested. There are also loopholes for Extremely low-volume manufacturers (Lamborghini, Ferrari, etc.), Kit cars and the like.
Michigan, the state I live in, does not currently require emissions testing, but, within 5 years, they probably will. As will other states without required emissions testing.
My point is this. Emissions testing is not a scary thing, unless you've swapped an engine into your vehicle that was not a factory option, or you have an MIL (Check engine Light) on.