When you work with Sitecore, there may be times when you need to update your search indexes. The search indexes are the files that are queries when website visitors do things like perform a website search, or, query for content within Sitecore itself. In the back-end, Sitecore works the Lucene search provider.
Lucene is a well-established search provider and is used world-wide. When working with a Lucene search index, Lucene creates a physical file on your hard drive for its indexes. Its these physically index files that Lucene uses to perform searches etc.. so if your Lucene index breaks, your search won't contain the latest files. If you find your search is missing certain information, then you may need to re-build your index.
The Lucene files are located within the Sitecore websites 'Data' folder, in the 'Index' folder:
If you want to have a look through your index files, you can use a tool called Luke For Lucene, to have a nosey. You can grab Luke here.
Re-build the Lucene Index
To re-build/clear the Index, the best option is to re-build the index within Sitecore. If you manually tried to delete them you might see strange errors within Sitecore so I wouldn't recommend it. To re-build your Sitecore instance, log into your Sitecore shell/dashboard by going to www.website.com/sitecore/shell/:
Select 'Control Panel':
Select 'Indexing Manager':
Here you will see a list of all the indexes. Select the ones you care about and hit the 'Build' button at the bottom of the dialog box. Your site should now re-index your search results and index :)
When you search your site, you should now see all your latest data contained.