VLC is a free software to play, transcode and broadcast video and audio files. It's a cross-platform software that support a big number of different formats and codecs.
A list of VLC possibilities is available here : http://www.videolan.org/vlc/features.php
VLC is present on the main Debian repository:
apt-get install vlc
It's possible to launch VLC by double clicking on its icon, or directly with the command line:
It's also possible to play a file directly from the command line interface, for example:
Debian specific information
Upstream specific information