How to change or remove the .html suffix of product and category URLs in Magento
When you add a new category or product in Magento it automatically creates a search engine friendly URL for that product or category based on its name and adds the .html suffix at the and of the URL. So, for example, if you add a product named HostKnox Sneakers, the URL displayed in the address bar of web browser that customers can use to access the page of the product will be domain.com/hostknox-sneakers.html. If you want to, you can change that suffix or you can remove it completely so that customer can access product and category pages by using URL addresses without a suffix (e.g. domain.com/hostknox-sneakers).
You can do this from the admin panel of your Magento. Go to System menu>Configuration, click on the Catalog button that's under the Catalog menu on the left, and select the Search Engine Optimizations panel on the right. After you expand the panel you'll see the options Product URL Suffix and Category URL Suffix. The default value of each is .html. You can change it to something else, or to remove it from URL addresses just delete it. Then click on the Save Config button in the upper right corner.
You'll also have to reindex the Magento data from System>Index Management.