aspect ratio is the term for how wide the image will be, the older TVs where 4:3 (4x wide by 3x tall also called 1.33) the widescreen TVs are 16:9 (1.78). The aspect ratio (AR) for movies has changed over the years and even then it is not necessarily consistent.

Now if the AR of the content does not match the display, then you need to make it fit. There are different alternatives at different stages. For example many displays have stretch or zoom functions that will help with filling up the screen. Studios can also use what is called Pan and scan, it is a bit like the zoom on the TV but in this case they pan on the film and pick the best part so unlike zoom on the TV, this does not centre on the image and someone decides what is the most important part to show. If you want to see the whole picture exactly the way it was meant to be shown (as seen in theatres, if we are talking movies, or on TV, if we are taking shows) then the only choice is to keep the original aspect ratio (OAR). But if the OAR does not match the AR of the display and you don't want anything lost or messed up then the only thing is to shrink the image to fit the screen and pad the rest with something, and since black is les intrusive they added black bars. Now the black bars will depend on OAR and AR of the display, if the OAR was wider then they will be top and bottom, if narrower they will be on the sides. For example Star Trek (The original show) was filmed in 4:3 so on a wide screen TV it has bars on the side, extremely old movies where also much narrower (1.33 for silent movies and 1.37 until 1953 when wider started being the norm..), most newer movies are either 2.39:1 or 1.85:1 so they will have black bars top and bottom. To complicate matters even a bit more some are filmed in 1.85 and composed not so important edges so that they can be shown in wider at the theatres (like 2.39) and narrower on TVs (1.78) so that at home you see a bit more of the top and bottom while in theatres a bit more on the sides.