Many video files are transferred and stored in a compressed format. Codecs are necessary to both compress and decompress the video files. A standard Windows 7 operating system will contain a codec pack that will include many common codecs such as, HEVC x265; x264; MPEG4 and MPEG2. HEVC (High Efficiency Video Coding) is x264 is actually just an encoder. Codecs may be lossy (the compression method will be imperfect and discard some original data) or lossless and allow for a perfect reconstruction.
When open a video file in Media Player or QuickTime you may get messages like these:
Such messages can be an indication that video codecs are missing. You can find out which codec you need for a video file using the free program, Video Inspector. This simple program will list the codec needed for any video file that you select with it.
In the first example, the selected file is a common MPEG 1 file and my Windows 7 OS already had the necessary codec. The second selected file was compressed with a Lagarith codec, that specializes in the lossless compression of RGB color.