Contrast ratio is the ratio between the brightest image a display can produce, and the darkest. The most logical way of measuring this would be to put a 100% white image on screen, measure it, then put a 0% black image on screen, and measure that. No company measures contrast ratio this way. Because there is no set or regulated way to measure contrast ratio, most companies significantly "fudge" their numbers. In most cases, the marketing department says the next display must have a certain "number" for contrast ratio, and the engineers figure out some way to create that number. Other times, the engineers aren't consulted at all.
The reality is, all contrast ratio numbers from manufacturers must be taken with a grain of salt, and should definitely not be compared. One company's 1,000,000,000:1 could easily be another company's 10,000,000,000:1, even though they may look the same.
In the end, sadly, all you can trust is your eye. In these days of brightly lit showrooms and sales floors, this is inherently misleading, as LCD TVs will appear to have a better contrast ratio in a brightly lit store, but in your home, a plasma will have a better contrast ratio.
For flat panels, plasma TVs are the best followed by local-dimming LED LCDs.