Search Engines are known to act upon websites that contain Duplicate / Similar content.
Your content could be similar to other websites on the Internet, or pages from within your own website could be similar to each other (usually the case with dynamic product catalog pages).
This tool allows you to determine the percentage of similarity between two pages.
The exact percentage of similarity after with a search engine may penalize you is not known, it varies from search engine to search engine, Your aim should be to keep your page similarity as LOW as possible.
Duplicate Content Filter
This article will help you understand why you might be caught in the filter, and ways to avoid it.
