When online you will encounter video which have been encoded using a variety of codecs. If you frequently watch different types of video you will generally have to have numerous different programs installed, each catering for different types of video - this can be an annoyance when you encounter a video for which you do not already have the required program or codec. Rather than waiting until the last minute, install the K-Lite Mega Codec Pack which includes a wide range of popular codecs for a number of different video formats.
As well as well known formats such as XviD, MPEG2 and DivX, the pack also includes audio codecs and a selection of alternative media players. Used in conjunction with each other, it is unlikely that you will encounter a video you are unable to play, and by using the alternative tools - such as Real Alternative - you can benefit from improved performance.
The codecs included in the pack can also be used when encoding your own videos and there are a number of handy tools which may well come in useful - such as a bitrate calculator, subtitling tool, video splitters and much more. The pack includes such a large number of tools and codecs, there is simply not space to cover them all in any details. Thankfully, it is not necessary to install all of the codecs at once if you do not need to - you can pick and choose the ones you need, adding others at a later date if necessary. There are a limited number of software installations that can be described as being essential, but this is one of them.
Note that this is the "Mega" version of the codec pack.