If there is no index or default www file in directories, httpd returns the contents of directory instead like this.

Index of /temp

	Name	Last modified	Size	Description
	Parent Directory		-
	temp.html	04-May-2012 05:10	300
	temp2.html	24-Nov-2012 09:20	200

Apache/2.2.26 (CentOS) Server at www.example.com Port 80

If you'd like to prevent the directory to be listed, you should reconfigure httpd.conf, here are the steps to do it:

1. Find the right directive to change.

If you want to forbid all directory listing, then find <Directory "DocumentRoot"> directive, for example:

<Directory "/var/www/html">
...
    Options Indexes FollowSymLinks
...
</Directory>

2. Remove "Indexes" like this.
<Directory "/var/www/html">
...
    Options FollowSymLinks
...
</Directory>

Or, add a minus sign before "Indexes".

<Directory "/var/www/html">
...
    Options -Indexes FollowSymLinks
...
</Directory>

3. Restart httpd

Now, you can try to list the directory again, this time, you should receive 403 error like this:

For more options of httpd server, please refer to the official document: core - Apache HTTP Server Version 2.4.