According to Wikipedia a codec is a program capable of performing encoding and decoding on a digital data stream or signal. The word codec may be a combination of any of the following: 'Compressor-Decompressor', 'Coder-Decoder', or 'Compression/Decompression algorithm'.

Ubuntu includes many codecs by default but it's not enough because there are many different multimedia formats. These multimedia codecs allow you to play audio, watch videos or listen to music of a specific format.

Open source multimedia framework provides a list of available codec plugins for GStreamer in different packages. Most of Ubuntu applications use this multimedia framework. However, other applications, such as Mplayer and Xine, do not use the Gstreamer framework. That means you have to install some extra codecs from the “Universe” and “Multiverse” repositories:

Ubuntu Help also shows us that to install extra codecs, press ApplicationsAdd/Remove..., type “codec” in the Search box and select which codecs you want to install. Press Apply to install the selected codecs.