Aspect ratio is the display's width to height ratio. Standard television has a 4:3 aspect ratio – it is four units wide by three units high. Most DTVs and all HDTVs have a 16:9 (widescreen) aspect ratio, more like a movie screen. The 16:9 aspect ratio is responsible for the great-looking widescreen content possible on an HDTV.
A widescreen set is perfectly suited for watching widescreen content, like HDTV, Blu-ray Disc™, and DVDs. And most widescreen sets can adjust standard content to fill a widescreen picture, whereas standard sets show widescreen content with black bars ("letterboxing".)
In a nutshell, contrast ratio is the amount of onscreen detail in blacks and other dark colors. It measures the difference of light between the brightest white and darkest black that can be produced.
For example, a static contrast ratio of 2,500:1 means that the brightest white is 2,500 times brighter than the darkest black. The larger the ratio, the more contrast there is between the dark and light detail shown onscreen.
There are two measurements of contrast ratio – static (a.k.a., native or onscreen) and dynamic. It is not important to know the differences between them, only which ratio is being reported so you make sure to compare either static to static or dynamic to dynamic.