Magento: How do you disable or change how a core observer method works

[gegrhr658]

I have been wondering about this for a while. What if Magento has written a core Observer class and performs a functionality that you do not want it to execute, or you want to rewrite it? Is there a way to say, don't use this method in an Observer, just use mine. If I setup a method for my own Observer won't it just do the core functionality first and then whatever I have implement?For example, Magento wants to save some data to the database in an Observer method, but I don't want it to save that data at all, I want it to save some other data that I have added columns or attributes to the database for.