What exactly is a kata?  Here's what I THINK it is.  "A warm-up exercise
to loosen the muscles.  It's a set of basic moves intended to be repeated
over and over."  Err, obviously, I'm missing a lot of information.  I'm
pretty sure all katas aren't basic, ne?  And are they really only a
warm-up exercise, similar to stretching?  Arigatou.

Katas are a series of martial arts moves blended together in a pattern, like
a dance.  The person doing a kata is essentially pretending to fight an
invisible opponent, or group of opponents.  Some katas are basic, with only
say, twenty or thirty basic movements, punches, kicks and blocks, and such
from beginning to end.  Some are much more complex, either with more moves,
or with more complex moves, or both.  Katas are practiced for the repetition,
so that the moves become instinctive.  Some Martial art forms use many
different Katas, and rely upon them heavily, including Katas with weapons.
Other forms use few or no Katas, being more freestyle.

Katas are generally used before a class to warm up the muscles.  They are
also used for practice outside of class.