Slides should be used during a presentation to support your message by emphasizing key points and illustrating important ideas. Unfortunately, they frequently become a needless distraction that can undermine your presentation.
The problem is that many presenters mistake slides for the presentation itself. Slides don't make presentations; people do. The sole purpose of slides are to support and reinforce your message. Rather than attempting to use slides to make your point by overloading them with bullet points and other information, use them to bring your words to life by highlighting key phrases and by introducing powerful and evocative imagery. Finally, don't fall into the trap of thinking of slides as something to be left behind after a presentation; that's what documents are for.
Alexei Kapterev provides excellent advice on how to produce effective slides in the following example:
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