Aspect ratios is a huge subject, and an interesting one.
From about 1930 to 1950 ish everything was shot in the 'academy ratio', this was about 1.35 :1 and is nearly exactly the same as 4:3 - old style tv.
The 60s and 70s saw a kind of 'wide war' with cinemas selling themselves over tv on that basis. There are lots of aspect ratios used in film but video is most often 4:3 or 16:9 reflecting normal and widescreen tv formats. CCDs in cameras are also 4:3 or 16:9 reflecting this.
It's not only the pixel ratio that determines the aspect ration, it is also the pixel aspect ratio - sounds more confusing than it is as all editirs have presets for the 2 aspect ratios in pal / ntsc / hdv.
To change aspect ratios you usually mask the picture.
I nearly always shoot 4:3 as my cam is native 4:3 and I find composition easier in 4:3.