For video playback, Ubuntu ships with the Totem player. Unfortunately, Totem is severely broken. It only plays Ogg Vorbis files. Even with the addition of codecs, it still doesn't play AVI or MPEG files correctly. That's in my experience.

That's why any new Ubuntu installation of mine always gets MPlayer. MPlayer has been around for quite some time, and is possibly the most complete media player for Linux.

To install MPlayer on Ubuntu:

sudo apt-get install mplayer

This downloads about 7MB of files from the repositories. This expands to 18MB of installed applications.

After installation, MPlayer becomes associated with video files. However, it's not the default. Right click on a video file and choose MPlayer to play it.

From the command line, you can also play movies by invoking

mplayer mymovie.avi

Once a movie is playing, MPlayer can be controlled via the keyboard.

<- and ->
Seek backward/forward 10 seconds.

up and down
Seek backward/forward 1 minute.

pgup and pgdown
Seek backward/forward 10 minutes.

[ and ]
Decrease/increase current playback speed by 10%.

{ and }
Halve/double current playback speed.

Reset playback speed to normal.

Pause (pressing again unpauses).

Step forward. Pressing once will pause movie, every consecutive press will
play one frame and then go into pause mode again (any other key unpauses).

q / ESC
Stop playing and quit.