If you have a widescreen tv, you have a 16x9 tv. Older Tv's that looked like squares were in the 4x3 aspect ratio. (4 pixels wide for every 3 pixels tall). Newever tv's and hdtv's are 16x9 (like yours) and are referred to as widescreen because they are a lot wider than traditional 4x3 televisions. 16x9 is popular b/c it is a ratio used often by movies (check out a movie screen next time you go, it's not a square like your old-fasioned tv, its a rectangle, like the new 16x9 tv's).
Read that FAQ for more info, but the above is sorta bare basics.
I realize that this was a basics question, so I hesitated to add this, but the aspect ratio of the set is not always directly related to the resolution capabilities. In most cases, you're right, but not always.
16:9 and 4:3 indicate a ratio of how wide a screen is to how tall it is. Let's say you have a widescreen and a non-widescreen tv that both have a screen that is 16" wide. The 16x9 screen would be 9" tall, whereas the 4x3 would be 12" tall (4:3 is the same ratio as 16:12).
by the way, this doesn't have anything to do with resolution or pixels; it's purely the physical ratio of width vs. height.