a series of moves performed in a routine and practised until perfected. often intended for application into a fight, however its usefulness is debatable.
similar to routines in gymnastics, they can be used in performances or competitions and notably in gradings, where they are judged on what they can do.
when performing the kata the student is often told to visualise the enemy attacking him or her and to visualise what they are doing in a combat situation. in this sense some people argue that they are effective in a combat situation. however on the flip-side people can argue that they are stiff and unchanging, unless you attacked in a certain way then the kata will be useless. really the effectiveness depends on how heavily a student taught the kata and the way they are taught it. if they are told to purely practise katas and not work on other ways of linking the moves or seeing different angles then they are stiff and would be far less effective in a fight. however if they they are encouraged to experiment and fit the moves together or be able to link any moves together in a multitude of ways then indeed there is a far more practical side to it.
after reading that previous section there is only really this to say, strict katas have very little practical use and are instead intended to be judges or to perform with the intention of entertaining.