There are many, many, filetypes for videoplayback. Most of you know AVI, WMV, MPG and TS files. It is important to know that those types are actually 'containers' they contain a video stream, an audio stream and data to keep these streams synchronized.
The streams can be encoded with many, many different codecs. This means that one AVI file may play correctly and the other doesn't. You could compare these containers to an archive format like ZIP, RAR, etc. (You can even 'extract' these containers, this is what happened a lot during the NLOTH Promo shows)
It is impossible to cover all videocontainers and codecs in this thread. However. If you follow these instructions you can play 90% of all videos on the net.
2. Common problems
We've al had a file we couldn't play without problems. Sometimes the video plays but audio is missing. Then you are missing the correct audio codec
If you hear audio but don't see video you miss the correct video codec. Simple logic.
As I said there are many, many codecs. You could install a single codec everytime you run in to problems. You could also installl a codec pack (not recommended, keep reading). Or you could use a single codec, which can handle more than 1 filetype.
3. Codecs on MS Windows
On Windows there is a central 'codec library' which is called DirectShow. A lot of mediaplayers depend on this system. Codecs for this system are called 'DirectShow Filters'. With the correct filters you can play everything in Windows Media Player.
On a clean install of Windows only a few filters are installed. Like WMV, WMA, MP3 and MS MPEG4 filters. For a lot of videos you'll need to install additional filters.
3. Examples of software setups which will playback most files
VideoLAN Player
This one just works. Can play most files out of the box, even broken and corrupted files. Must have.
Media Player Classic - Homecinema
Plays a lot out of the box (less than VideoLAN though) but interface is a bit easier than VideoLAN. With additional codec packs you can make it at least as good. Has a lot of advanced options. It even supports Hardware acceleration for HD videos. Although only recent video cards are supported. Does use the DirectShow system.(My personal favorite).
Windows Media Player
The standard player on Windows. A decent player but lacks a lot of codecs/filters. But with the following filters you can play most files!
* ffdshow (with support for basicly every audio/video codec)
* Haali Media Splitter Necessary for containers used in a lot of HD videos around the net.
4. Other video formats
Unfortunately there are still video codecs which are not as open as the others. For some videos you will need QuickTime or RealPlayer. Luckily the newer versions of those videoformats are supported by other players as well. But a lot of older clips are not playable without these players (or alternative illegal filters)
5. About codec packs
Some are better than others. I strongly advice you to stick with ffdshow with Haali Splitter. Codec packs install a lot of filters and can cause stability issues which can't be resolved easily. I am sure there are packs without these problems but use them with care. Don't install more filters than necessary!