What were the reasons for the Congress of Vienna?

The Congress of Vienna was convened in 1814.  Its major
purpose was to set up an international order for Europe after the Napoleonic Wars.  In
other words, the Congress met to decide things like which lands would be controlled by
who and to set up the "rules" by which European affairs would
run.

As an example of the first type of thing, the Congress
set up the Netherlands as a new country and they created the states of Genoa and
Piedmont in Southern Europe.  They also set new boundaries for various countries like
Prussia.

As an example of the second, they restored to the
throne various dynasties that had been overthrown by Napoleon.  In doing so, they were
essentially saying that Europe would go back to being a region of
monarchies.

Overall, the point of the Congress was to try
to set up a system whereby Europe could be at peace.  The Congress tried to do this by
setting new boundaries and by doing things like reestablishing the monarchies in hopes
that that would prevent further wars within countries.