The question "Why are there so many different media formats?" would detain me for hours of ranting. However, I have recently found a website which is very useful for most file conversions.
media-convert.com really is an excellent little FREE site. It operates all on page. You browse to enter the file you want to convert, be it video, audio, image, document. Then you enter the format you want the file in. Hit the button. It takes a few seconds and then your file is ready to download in the format you want it.
After spending years downloading individual bits of trial software (some with annoying watermarks etc unless you hand over cash) it is great to find just one free site to cover most eventualities. I found the site yesterday when I was horrified to see that vidoes on my mobile were in "3gp" format - of which I had never heard. I tried various trial software packages to get the 3gp files into avi format but they all had watermarks or blacked out when inserted into Powerpoint. Then I found this site and "bingo". It even coverts pdf files back and forwards, an exercise with which I have often struggled.