LaunchServices
This option rebuilds the LaunchServices, meaning it removes the links between documents and applications which can open them by default.
When you launch a new type of document for the first time, you are presented with an alert message. You then have several options: allow your document to be opened by the application to which the System defaulted, let the Finder display this application in its surrounding sub/folder, or cancel opening the document. Once opened a first time, the link between this type of document and the application which executed it is stored. Also stored are all installed applications, your choices of specific applications to open specific types of documents as well as your choice of a default web-browser.
After rebuilding, double-clicking documents or web-links may call unexpected applications. To change that back to your preferred settings, select a file of the desired type in the Finder, choose "File/Get Info" (Apple-I), select the application you prefer from the list of available choices in the "Open With..." menu, then click "Change All" to tell the Finder that all similar files shall call that same application. The default web-browser can be redefined in any browser's Preferences pane. Rebuilding LaunchServices also allows you to correct misc problems: Duplicated display or display of deleted applications in the contextual menu's "Open With", incorrect Icons for certain files and folders, launching of wrong application on double-clicking a document.
LaunchServices can be rebuilt from the Maintenance panel > Rebuild or from the Automation panel.
Warning: Rebuilding Launch Services takes quite a while to execute. Please wait!