http://www.htaccess-guide.com/

the easy way is to put a blank index.htm and index.html and for some sites main.htm and main.html, or put all four. When someone goes to that folder, all they will see is a blank page, unless they have access to the actual file in the folder itself.

for instance, if I send you to a folder with pictures of my backyard, you will see nothing other than some warning page.

http://glenburniemd.net/backyard/

If instead I send you direct to a picture in it;

http://glenburniemd.net/backyard/garden.jpg