When you download video from the internet you need to install codecs to allow you play them. Instead of installing codecs one at a time as you come across each different format it is much easier to install codec packs. This will install many codecs all at once.
K-Lite Codec Pack
One the best codec packs is the K-Lite Codec Packs which come in three different versions . If you are looking to only play back video and you are not going to be involved in creating your own video for the web and uploading we suggest you grab just the standard codec pack.
You can download the K-Lite Codec Packs here or here .
Combined Community Codec Pack
CCCP is a small and compact codec packs that contains only the bare minimum of what is needed for most movie and tv show videos that you will download. For most users this pack is a perfect solution.
It is important to note that this codec pack intentionally disables support for many codecs it considers unnecessary. It does this with the best of intentions as Codecs often clash when they are not selected properly.
You can download the CCCP Codec Packs here
Ace Mega Codec Pack
Is a comprehensive codec pack that has about everthing but the kitchen sink and although it is an overkill for most users you can download it here