In the world of physical pictures we have a few different media: Photos, slides, negatives, etc... and to look at a slide requires very different technology than to look at a photo.
This is similar to different file formats. You may have a bunch of image files, but you need a different bit of technology to read a .jpg instead of a .bmp or a .png or a .psd. Sure, they're all images, but the image is contained in a different "wrapper," and the computer needs a different line of code to be able to show them to you. These bits of code, languages, wrappers, what-have-yous are called codecs.
You must have the correct codec reader installed on your computer to open a file and manipulate it. Last night I got a call from a friend who couldn't get her Word file to open. That's because she had an older version of Word that only had a codec for .doc files, but the file she was trying to open was saved in a .docx codec. I helped her install a new codec reader so she could open the file and get back to working on her paper.
This also happened with some video files. We captured some footage to a .mov codec. But there is more than just one type of .mov file. These video files happened to be in a .mov wrapper that included an HDV codec. Once these files were transfered to another computer without an HDV .mov codec reader, the files were useless until they had been put through a conversion program that repackaged them in the right wrapper.
The computer must know what kind of wrapper the file is in, and it must also have a way to read that file.
If you don't have the right codec, your file won't open.
Your Media Production Mentor