The aspect ratio of a picture refers to the width in relation to its height.
Most common aspect ratios used are: 4x3 & 16:9 (four by three and sixteen by nine)
Aspect ratio is expressed width x height. For example, a picture that has an aspect ratio in 4x3 means that the picture is 4 units wide and 3 units high.
Different aspect ratios
Has been the standard television format of the 20th century.
This format became the new standard with the introduction of the widescreen TVs and high definition video.
Other aspect ratios are rarely used like 21x9, for instance in cinematic releases or for LCD Cinema displays. The aspect ratio is not standardized.
Most common resolutions
For broadcasting the standard is in countries that use PAL: 720 x 576 and countries that use NTSC: 720 x 486. (List for countries that use NTSC and those that use PAL you can find here)
Sometimes your 16x9 picture will look like it is pressed into a 720 x 576 window. That is because your16x9 export is anamorphic which means that the pixels will behave differently in different environments, so the picture will adapt to the resolution used by your tv screen. When uploading on YouTube you can paste this code: yt:stretch=16:9 in your tags so YouTube will display the video in proper 16x9
Here I will give you the most common resolutions used for 4x3 and 16x9 picture standards, expressed in pixels, for use on PC.
720 x 576
640 x 480
512 x 384
1024 x 576
640 x 360
512 x 288
Most decent video editors are capable to convert from one aspect ratio into another. Note that parts of the image will be cropped or you can choose for black side bars.