Most servers are configured so that directory browsing is not allowed, that is if people enter the URL to a directory that does not contain an index file they will not see the contents of the directory but will instead get an error message. If your site is not configured this way you can prevent directory browsing by adding this simple line to your .htaccess file:
IndexIgnore */*
But there may be times when you want to allow browsing, perhaps to allow access to files for downloading or for whatever reason, on a server configured not to allow it. You can override the servers settings with this line:
Options +Indexes
Easy!