The ellipses is a very useful tool. It is used to tell the reader when something is being intentionally left out by the author.
It can be used for pauses in thought or conversation, but these pauses should be considerable. If the pause is naturally in a sentence, it should be indicated with a comma. I have seen people pretty much replace the comma in their work with ellipses. Too much ellipses use can be very distracting to the reader.