Disallow and noindex are two ways to tell search engines not to index a specific page or set of pages on your website. Disallow will tell the search engine crawler not to visit the specified pages, while noindex will tell the crawler not to index the specified pages but still visit them. Which you use depends on your goals. If you want the crawler to completely ignore the specified pages, use disallow. If you want the crawler to visit the pages but not index them, use noindex. Keep in mind that using either disallow or noindex will still allow people to reach the specified pages if they have the direct URL. If you want to prevent people from reaching the specified pages altogether, you'll need to use additional security measures such as password protection.