I understand that the way a TV picture tube and computer monitor works
is that electrons are fired at a phospherous surface ligthting up to
show an image.Can you explain this process and also why does a magnet
disrupt or ruin the TV tube. And does a plasma TV work the same and
will a magnet harm a plasma TV?

http://entertainment.howstuffworks.com/tv.htm/printable
should tell you all you want about tv... there's a link off the bottom
for plasma displays and other techs.

The reason a magnet disturbs the image is that a magnetic field exerts
a force on moving charges, such as moving electrons. In fact,
electromagnets are used to steer the beam in order to draw the
picture. Strong permanent magnets can cause changes to the magnetic
properties of the TV components, resulting in a persistent change
(usually bad) to the performance of the tube.