In the earlier versions of Windows 7, to play some media files, one had to download and install codec files or use alternative media player such as QuickTime, VLC, etc.
But in Windows 7, you will be able to be able to play DivX, XviD & QuickTime .mov files without installing any codecs or using alternate media players.
Microsoft has added built-in native DivX and XviD support to playback movies & video files encoded in these formats.
Simply double click the media files to play them.
In case you do not know what are Codes, a Codec is a combination of Coder and Decoder or Compressor and Decompressor. And technically a codec is software that is used to compress or decompress a digital media file, such as a song or video. Windows Media Player and other programs use codecs to play and create digital media files. There are codecs for many purposes – for example, audio, video, xvid, DVD, etc.
In other words, a Codec is software that is used to compress or decompress a digital media file, such as a song or video. There are hundreds of audio and video Codecs in use today. Some have been created by Microsoft, but the vast majority of Codecs have been created by other companies, organizations, or individuals.